The novel's title character and protagonist, Martin is a curious young man whose life in the medical profession makes up the plot of the book. He is stubborn and inclined toward laboratory science, rather than the practice of being a physician. He has opposing characteristics and can be both cold and compassionate, both driven and easily swayed. Furthermore, he is a romantic at heart.
A country doctor in Elk Mills, Doc Vickerson was Martin's first introduction to the world of medicine. The doctor is not an all-together educated man although he is supportive of Martin. And, although Doc Vickerson is an alcoholic, he is not altogether unlikable.
A German Jew, Max Gottlieb is Martin's mentor. He is a scientist rather than a physician. He is often seen as eccentric and as cold or lacking in compassion although he does have a deep belief in Martin. A patient man, he is utterly driven by a search for "truth" and is fully committed to the study of science.
A graduate student at Winnemac University, Madeleine Fox attends the same college as Martin. She eventually becomes his fiancé, but is what Martin calls an "improver." She is a snobbish student of literature who tries to change people, including Martin, to fit her beliefs and her society.
One of Martin's fellow medics, Fatty Pfaff tries his hardest to learn and yet he does not have the brightest mind at Winnemac. In fact, Fatty is the archetypal nice, yet dumb, guy at the University. He eventually becomes an obstetrician.
A classmate of Martin's, Ira Hinkley is a preacher who tries to impose his religious beliefs on others. His path crosses Martin's more than once and he eventually betrays Martin. He believes he is doing "good" but is arrogant in his beliefs—most of which are narrow-minded, superior-minded, and colonial.
A good student and successful surgeon, Angus Duer was Martin's rival at medical school. He is a hard worker and always achieves that for which he strives. He believes in success and is the opposite of Martin in many ways.
The class jester, Clif Clawson, resigns from medical school and becomes a car salesman. He is unorthodox in his business and, in the end, too different from Martin to remain his friend.
Martin's loyal wife, Leora is opinionated and yet completely supportive and understanding of Martin and his career. She is caring and although ambitionless herself, she is loving and a perfect fit for Martin.
The compassionate physician, Dean Silva is supportive of Martin and epitomizes the "good doctor." He is caring and believes more in the practice of medicine than in research.
The director of the McGurk Institute and later a leading member of the League of Cultural Agencies, Tubbs stands for everything that Martin opposes. He believes wholeheartedly in competition and, not in the individual, but in "cooperation." He is one of the many "salesmen" in the medical industry that exist in the novel. He is criticized by Martin for not being as intelligent as someone in his position should be.
Neither a laboratory man nor a physician exactly, Sondelius is a one-man army against disease. A captivating speaker, he is compulsive and adventuresome, and yet, often compassionate.
The subject of a great deal of Lewis's satire, Pickerbaugh is the Director of the Department of Public Health in the small city of Nautilus. He campaigns for cleanliness and writes bad poetry about Health and sanitation for his daughters (whom he has named the "Healthette Octette") to sing. He is a great salesman and commercialist.
The oldest daughter of Almus Pickerbaugh, Orchid is a young, flirtatious, and beautiful girl who Martin becomes enamored of Martin and admires his eccentricities and intelligence.
A department head at McGurk, Holabird has very much the same business mind-set as Tubbs. He believes in competition and success and eventually joins the League of Cultural Agencies with Tubbs. He constantly brags about an old war wound and becomes another foil for Martin's personality.
The secretary to Dr. A DeWitt Tubbs at the institute, Miss Pearl Robbins is a beautiful woman who joins the men in competition. From being Tubbs's assistant, she has learned the "business" and even attempts to attain the directorship when Tubbs resigns. When Gottlieb is head of the institute, she basically runs it because of Gottlieb's lack of attention in commercial matters.
A laboratory scientist much like Gottlieb, Terry Wickett is completely committed to his work. He does not have a family or much of a social life because of this commitment. He is the extreme of what Martin could be, and by the end of the novel he becomes the symbol of freedom and independence. He is very much an individual in a world of "collaborators" and he strives always for what he believes. He is stubborn and thought of as cold, though he and Martin have an "understanding" of each other and are, in fact comrades by the end.
The second wife of Martin Arrowsmith, Joyce Lanyon is a rich woman with whom Martin does not have very much in common. She is a high class "arranger" and a widow who spends her time frivolously and goes to Africa to "save primitive art."