The inhibitory effect of Oncovin (vincristine sulfate) on cell division was studied in human cell line NHIK 3025. Oncovin arrested the cells in metaphase at concentrations as low as 10(-3) mug/ml. At 8 X 10(-3) mug/ml and higher concentrations, the arrest was complete after 6 hr of treatment. The arrest was irreversible after exposure for 6 hr to 16 X 10(-3) mug Oncovin per ml. The X-ray radiosensitivity of aerobic cells of the same line pretreated with 16 X 10(-3) mug Oncovin per ml for 6 hr (Oncovin removed before irradiation) was found to be about equal to that of untreated cells. Even when present during irradiation, Oncovin did not exert any modifying effect on the radiosensitivity of either aerobic or extremely hypoxic cells.