Highly purified peripheral blood lymphocytes from healthy individuals were obtained from samples collected before and after a standardized bicycle ergometer test. The physical activity resulted in a marked increase of circulating lymphocytes. The proportion of T lymphocytes estimated as cells forming rosettes with sheep red blood cells after incubation in the cold decreased, whereas a corresponding increase of cells with receptors for C3, IgG-Fc or surface immunoglobulin was noted. Moreover, after work an increase of cells simultaneously reacting with cell surface markers usually designed as T- or B-cell markers occurred. The reactivity of lymphocytes collected after work in response to Con A, PHA, PWM and PPD was impaired, whereas the slight response to LPS was unchanged. The K-cell cytotoxicity of lymphocytes collected after work increased. The data indicate that physical activity leads to the mobilization of lymphocytes from as yet undetermined sites and with changed composition and reactivity.