The effect of cimetidine on survival was investigated in mice infected with herpes simplex virus type 2 (HSV-2), murine encephalomyelitis virus (GD-VII), and vesicular stomatitis virus (VSV). BALB/c mice, 5 weeks of age, were injected intraperitoneally (i, p.) with 5.5 ×10⁵ plaque-forming units (PFU) of virus/0.5 ml, and cimetidine (1 mg/0.5 ml) was administered simultaneously. The survival rates of 80% and 85% in the cimetidine groups were significantly greater than the 10% and 23% for the control groups. The GD-VII-and VSV-infected control mice were dead at 3 days after virus inoculation. However, more cimetidine-treated mice survived than control mice. When anti-mouse T-cell serum or cyclosporine, which is a helper T-cell suppressor, was administered to BALB/c mice; the effect of cimetidine against the HSV-2 infection could be observed. When injected with anti-asialo GM₁, BALB/c mice or beige mice with low natural killer (NK) cell activity were not affected by cimetidine. Lastly, cimetidine was shown to activate the cytotoxic action on NK cells. The above results indicate that the antiviral effects of cimetidine depend on NK cell activation.