Blood mononuclear cells from 20 healthy untrained volunteers were isolated before, during, 2 hr and 24 hr after bicycle exercise at 80% of VO2 max for 1 hr. A reverse plaque forming cell assay was used to investigate B-lymphocyte function. Stimulation with pokeweed mitogen, recombinant interleukin 2 and Epstein-Barr virus resulted in significant decreases in numbers of IgG-, IgM- and IgA-secreting blood cells during as well as 2 hr after exercise, with reversal to pre-exercise values 24 hr later. During and after physical activity we found an unchanged concentration of CD20-positive B lymphocytes suggesting that the suppression of immunoglobulin secreting cells was not due to changes in numbers of B lymphocytes. A decline in CD4/CD8 ratio was measured only during exercise with normalization after exercise. Therefore the B-lymphocyte suppression, most pronounced 2 hr after exercise, was presumably not due to changes in T lymphocytes also indicated in the experiments using EBV-stimulated cultures since EBV acts directly on B lymphocytes. 2 hr after physical activity an increased level of CD14-positive monocytes were observed and the monocytes expressed higher levels of surface HLA-DR during as well as 2 hr after exercise. Addition of indomethacin caused an increased response only in the IL-2 stimulated cultures, suggesting that IL-2-sensitive activated B lymphocytes are downregulated by prostaglandins. Purified B lymphocytes produced plaques only after EBV-stimulation, and in these cultures no exercise-induced suppression was found, likewise suggesting an inhibitory effect of the activated monocytes.