Effects on the immune system of 6-min "all-out" ergometer rowing were investigated over 2 days (2 x 3 bouts) in eight male oarsmen with a maximal oxygen uptake of 5.5 +/- 0.1 l/min (mean +/- SE). Blood samples were obtained before, during, and 2 h after each bout and on the day after the last bout. Compared with levels at rest, the first bout of exercise increased the concentration of leukocytes (2-fold); neutrophilic granulocytes (2-fold); lymphocytes (2-fold); monocytes (2-fold); the blood mononuclear cell (BMNC) subsets CD3+ (2-fold), CD4+ (2-fold), CD8+ (3-fold), CD16+ (8-fold), CD19+ (2-fold), and CD14+ (2-fold); the NK cell activity (2-fold); and plasma interleukin-6 (3-fold) (P < 0.05). During the last bout even higher levels were noted for leukocytes (3-fold); neutrophilic granulocytes (3-fold); lymphocytes (4-fold); the BMNC subsets CD4+ (3-fold), CD8+ (5-fold), CD16+ (13-fold), CD19+ (5-fold), and CD14+ (3-fold); and for the NK cell activity (4-fold) (P < 0.05). During the recovery periods all values were at or above the level at rest, and elevated concentrations of leukocytes (38%), neutrophilic granulocytes (48%), and lymphocytes (46%) reflected in the BMNC subsets and increased NK cell activity (119%) were also noted on the day after the last bout (P < 0.05). The results show that maximal exercise with large muscle groups provokes higher immune responses during repetitive bouts.