1. Short-lasting strenuous physical exercise leads to a marked increase in the blood concentration of leucocytes in healthy humans. The cell sources of this leucocytosis have not been definitely identified, neither has the contribution made by the various subtypes of leucocytes.
2. We have compared the exercise-induced leucocytosis in six splenectomized and six intact subjects. The blood concentrations of leucocytes as well as of most subtypes, except monocytes and eosinophilic granulocytes, increased substantially at the end of a 5 min exercise (200 W) bout.
3. The increases were similar in the splenectomized subjects and in the control subjects. Thirty minutes after completion of the exercise the various leucocyte blood concentrations had returned to pre-exercise levels.
4. The plasma concentrations of several cytokines (interleukins-1α and −6; tumour necrosis factor-α and granulocyte colony-stimulating factor) known to affect leucocyte kinetics did not change appreciably with exercise in either group.
5. We conclude that exercise-induced leucocytosis can take place in the absence of the spleen. There was no evidence that cytokines known to affect leucocyte formation, mobilization and activity are involved in this leucocytosis.