In this work T-enriched lymphocyte populations (obtained from Ig-anti-Ig-Degalancoated columns) were studied for their surface markers, especially for Fc and complement receptors, before and after a pulse-PHA-stimulation. It has been demonstrated that activated T lymphocytes express on their surface both of these markers in high percentage. The hypothesis is discussed that B and T lymphocytes differ only with regard to the presence of surface immunoglobulins or the ability of forming spontaneous rosettes. On the contrary the other surface markers may probably be expressed when needed functionally by the cells.