This study examined the impact of heat exposure and moderate, intermittent exercise on the CD16+ and CD56+ cell counts and cytolytic activity. Eleven healthy male subjects [mean (SD): age = 27.1 (3.0) years, peak oxygen intake, VO2peak = 47.6 (6.2) ml. kg-1. min-1] were assigned to each of four different experimental conditions according to a randomized-block design. While in a climatic chamber maintained at a comfortable temperature (23 degrees C) or heated (40 degrees C, 30% relative humidity, r.h.), subjects performed either two 30-min bouts of cycle-ergometer exercise at approximately 50% VO2peak (separated by a 45-min recovery interval), or remained seated for 3 h. Blood samples were analyzed for CD16+ and CD56+ cell counts, cytolytic activity and the concentrations of various exercise stress hormones (norepinephrine, epinephrine and cortisol). Heat exposure alone had no significant effect on cytolytic cells. The (CD16+ and CD56+) cell count increased significantly (P < 0.0001) during each exercise bout under both environmental conditions, but returned to baseline levels 15-45 min following each exercise bout. Total cytolytic activity (determined by a standard 51Cr release assay using K562 cells) followed a similar pattern, but cytolytic activity per CD16+ or CD56+ cell was not significantly modified by exercise. Our findings show a strong association between hemodynamic factors and recruitment of cytolytic cells into the peripheral circulation. Alterations in cytolytic activity of the whole blood during and following moderate exercise seem to be the result of changes in CD16+ and CD56+ cell counts.