In the study the effect of in vitro ozone exposure on human peripheral blood natural killer (NK) cell activity measured against K562 tumor cells has been examined. The data showed that NK activity was inhibited in a time dependent manner with marked suppression observed after 6 hours of ozone exposure. Significant inhibition occurred at three different levels of ozone exposure (1.0, 0.5 and 0.18 ppm) and E/T ratios of (50:1, 25:1 and 12.5:1). The ozone effect was not merely a nonspecific toxicity as NK activity recovered within 18 hours following exposure either spontaneously or by stimulation with IL 2. Moreover, unexposed cells treated with supernatant from ozone exposed cells showed no decrease in NK activity. A direct effect of ozone on the natural killer cells was suggested by the finding of suppressed activity in an NK enriched population. Since conjugate formation of the ozone exposed NK cells with K562 targets was unchanged the authors assume that the inhibitory effect of ozone was at a post binding step in the lytic pathway. (Copyright (c) 1990 by Hemisphere Publishing Corporation.)