Peripheral blood lymphocytes obtained from 16 controls, 16 untreated cancer patients and 15 cancer patients treated with irradiation were cultured with phytohemag-glutinin (PHA) and pokeweed mitogen (PWM). The cultures were terminated on 3 and 7 days of incubation, and mitotic rates were obtained. The mitotic rates of untreated cancer patients for both mitogens did not differ significantly from those of controls. However, the mitotic rate obtained for 3-day PHA culture of the radiation-treated cancer patients was significantly different from controls, whereas the 7-day mitotic rate stimulated by PWM was significantly lower than the untreated cancer patients and controls.