Functional and phenotypic immunological parameters were examined before, at the end of, and 20 minutes after the induction of positive and negative mood states, varied for arousal level, and a neutral state. The subjects were 14 male actors who experienced each condition on a different day. Compared with a neutral condition, all mood states affected several immune parameters (e.g., natural killer cell percentage and activity and percentage of suppressor/cytotoxic T cells), regardless of the valence or arousal level of the mood induced. The only immune variable differentially sensitive to positive and negative mood states was the proliferative response to the mitogen phytohemagglutinin; the response increased after positive moods and decreased after negative moods. Analysis of covariance for repeated measures indicated that heart rate, alone or in combination with physical activity and cortisol levels, had an impact on mood effects for most of the immune parameters investigated.