The natural killer (NK) activity of peripheral blood lymphocytes from 18 cancer patients was studied prior to and after multiple administration of organo-germanium compound (Ge-132). In successive oral administration of Ge-132 at a dose of 1000 mg/day for 10 days, NK-activity of patients was augmented at 3 days, but by 10 days, depression of NK activity was observed in all cases. In intermittent oral administration of Ge-132, however, more than half of the patients with augmented NK activity at day 3 maintained the high activity level at day 10. This result suggests the superiority of intermittent administration of Ge-132 for clinical use.