It was previously reported that red ginseng extract inhibited carcinogenesis by urethan, DMBA and aflatoxin B₁, (Cancer Detection and Prevention, 6: 515-525, 1983). In an attempt to investigate the mechanism of the anticarcinogenic effect of ginseng, the N.K activity and the incidence of lung adenoma were followed over a period of 48 weeks post injection with urethan or benzo(a)pyrene. The N.K activity was markedly depressed from 4 weeks to 24 weeks after injection of carcinogens. This decreased N.K activity was returned to the level of controls by administration of ginseng. At the same time, lower incidence of lung adenoma noted following administration of ginseng to urethan-injected mice. However, the lung adenoma induced by benzo(a)pyrene began to occure at 48 weeks in which N.K activity had naturally declined to a level too low to be affected by ginseng, and administration of ginseng did not decrease the incidence. In conclusion, these results suggest that the anticarcinogenic effect of ginseng may be related to the augmentation of N.K activity.