Twenty-one young male subjects exercised on a cycle ergometer for 60 min at 60% of VO2max. Blood samples collected every 30 min throughout exercise and continuing to 120 min recovery served for the immunological tests. Exercise induced biphasic changes in the various leucocyte subsets. There was a granulocytosis, lymphocytosis and monocytosis during exercise, and a further granulocytosis and a slight monocytosis, but a lymphocytopenia during recovery. All lymphocyte subsets (CD3+, CD19+, CD4+, CD8+, and CD16+ cells) increased in number during exercise, were decreased 30 min after exercise, and had not returned to baseline levels by 120 min of recovery. The apparent lymphocyte responsiveness to the mitogens phytohaemagglutinin (PHA) and pokeweed mitogen (PWM) declined significantly during exercise, returning to normal by 120 min of recovery. The natural killer (NK) activity rose markedly during exercise, but decreased to almost half the pre-exercise level at 30 and 60 min of recovery, returning to baseline levels after 120 min of recovery. Functional capability correlated well with the percentage of each major responder subset in the assay, suggesting that the in vitro lymphocyte PHA- and PWM-responsiveness and the NK activity did not change significantly on a per cell basis. The analysis of lymphocyte marker antigen density revealed that the CD3+, CD4+, CD8+ and CD19+ lymphocytes mobilized into the circulation during exercise did not express the respective CD3, CD4, CD8 and CD19 molecules as strongly as did the subsets circulating at rest, whereas the expression of the CD16 antigen on CD16+ lymphocytes remained unchanged.