Alcohol consumption is associated with increased morbidity and mortality related to infectious diseases and malignancy (1-5), although immune mediation of these relationships is controversial. Specifically, the activity of natural killer (NK) cells, which are involved in the resistance to infections and metastasis, can be suppressed in the presence of ethanol in vitro. However, acute consumption or infusion of ethanol in vivo exerts no effects on NK activity assessed in vitro thereafter. Therefore, we have developed and used a method to study the effects of ethanol on NK activity in living rats by using an NK-sensitive metastatic process and selective depletion of NK cells in vivo. Acute ethanol intoxication caused a marked suppression of NK activity in vivo and a tenfold increase in the number of MADB106 tumor metastases. Ethanol had no effect in rats selectively depleted of NK cells or when an NK-insensitive tumor (C4047) was used. These findings suggest that even acute ethanol intoxication markedly suppresses NK activity in the living organism. This suppression may underlie some aspects of the association between alcoholism, infectious disease and malignancies.