This experiment was conducted to clarify the influence of laughter on the immune system. Nineteen volunteer subjects were taken to a variety theater to experience laughter for three hours. Blood samples were taken from the subjects immediately before and after the performance. The NK activity and CD 4/8 ratio of these blood samples were examined. Without exception, in those subjects with NK activity levels which were below average before the performance, there was a significant increase in these activity levels, and in the CD 4/8 ratiosimmediately after the performance (p<0. 05,Wilcoxon's rank-sum test). In all the subjects who had CD 4/8 ratios that were above the standard level immediately before the performance, there was a significant decrease of these ratios immediately after the performance (p<0. 05,t-test). From these findings it is concluded that laughing increases the NK activity of people whose ac- tivity levels are below average and normalizes the CD 4/8 ratios of people whose ratios are above or below the standard levels.