Fig. 4. NK cell activity in African American controls () and alcoholics (•). Data are presented as mean ± SEM percentage of specific cytotoxicity at the 20:1 E:T ratio. For NK cell activity, there was a group effect (F(1,39) = 5.6, p < .02) and a time effect (F(2,78) = 3.0, p < .05), but no group by time interaction. In the controls, there was a significant increase of NK cell activity from 23:00 to 06:30 hours (p < .05); the alcoholics showed no change across the nocturnal period.