The studies were performed on healthy well-trained cyclists. Maximal physical exercise was performed on a Monark bicycle ergometer according to individual schemes. Heart rate amounting to about 200 bts/min and oxygen consumption stabilization were considered as criteria for maximal physical exercise. In this study we have investigated the effect of short-term stimulation of conditioned sportsmen with thymic hormones and evaluated T cell subsets, DR antigen and transferrin receptor expression as well as mitogen-induced proliferation of lymphocytes before and after maximal physical effort. The results suggest that intensive physical exercise may be responsible for transient decrease of CD4/CD8 ratio and mitogen responsiveness, and increase of mononuclear cells number bearing HLA DR + and CD71 antigens. These changes were modified by the treatment with thymic hormones.