Maximal physical exercise was performed on a Monark bicycle ergometer according to individual schemes. The investigations were carried out on 11 highly trained cyclists, aged 20 +/- 1 years. Heart rate (HR) amounting to about 200 bts/min and oxygen consumption stabilization were considered as criteria for maximal physical exercise. Lymphocyte phenotypes were determined using monoclonal antibodies. A significant increase in Ts (suppressor, cytotoxic) and a moderate increase in Th (helper, inducer) and NK (natural killer) cell numbers were noted 3 min after maximal physical exercise. At the same time, a significant diminution of the Th/Ts ratio was observed. A significant increase of interleukin 1 production and a diminished interleukin 2 production as well as spontaneous interleukin 2 receptor expression (Tac antigen) were observed at the same time. After a 2-h recovery, there was a normalization of most of the parameters investigated. The results suggest that maximal physical exercise in highly trained bicycle racers generates transient changes in immune cell function.