Patients with psoriasis receiving cytotoxic drug therapy and appropriate control subjects were exposed to (1) sensitizing doses of delayed allergens, dinitrochlorobenzene (DNCB), and paranitrosodimethylaniline (NDMA); (2) evocative patches of pentadecylcatechol (PDC); and (3) immunizing doses of a plague vaccine. Psoriatic patients so treated were very difficult to sensitize to DNCB and NDMA, reacted normally to PDC, and generally failed to give a primary or a secondary immune response to plague antigen.