The present study was designed to test the hypothesis that the exercise-induced changes in blood mononuclear cell (BMNC) subsets, BMNC proliferative responses and lymphokine activated killer (LAK) cell activity are mediated by increased epinephrine concentrations. Healthy male volunteers 1) exercised on a bicycle ergometer (75% of VO2max, 1 h) and 2) on another day were given epinephrine as an intravenous infusion to obtain plasma epinephrine concentrations comparable with those seen during exercise. Blood samples were collected in the basal state, during the last minutes of exercise or epinephrine infusion and 2 h later. During both perturbations the %CD3+ and %CD4+ T cells declined and the %CD16+ NK cells increased. Two h afterwards the CD14+ monocytes increased, while no changes were observed in %CD8+ T cells or %CD20+ B cells. The phytohemagglutinin (PHA) response declined during both epinephrine infusion and exercise experiments. The changes in interleukin-2 (IL-2) effect on proliferation and cytotoxic activity (LAK cell activity) were more pronounced in exercise experiments than during epinephrine. Exercise and epinephrine caused increase in concentrations of lymphocytes and neutrophils, but the changes were more pronounced in exercise experiments. The results indicate that, in response to physical exercise, the rise in plasma epinephrine may contribute to the changes in cellular immunity.