NK cell activity in beige mice was compared with that in control BALB/c mice and BALB/c mice treated with HSV-2, sarcoma-180 cells, cimetidine and cyclosporine. The NK cell activity in five week-old beige mice was lower than in three week-old BALB/c mice. The NK cell activity in eight week-old BALB/c mice was four to eleven times the activity in BALB/c mice that were three weeks of age. BALB/e mice, injected with 2 mg cyclosporine or 10⁷ plaque forming units (PFU) of HSV-2, had decreased NK cell activity. BALB/c mice, injected with 2 mg cimetidine, 10⁶ sarcoma-180 cells and 6.0×10⁵ PFU of HSV-2, had high NK cell activities compared with the control mice. Cimetidine further increased the effect of HSV-2 or sarcoma-180 on NK activity. BALB/c mice that received a transfer of less than 10⁷ NK cells developed a high resistance against viral infections, but mice that received more than 10⁷ transferred NK cells had a lower resistance to viral infections than the control mice or mice receiving less than 10⁷ NK cells.