The present study was carried out to investigate the influence of cryopreservation on human T-cell subsets defined by their membrane receptors for Fc IgM (TM) and Fc IgG (TG) and by their membrane antigens. For this purpose isolated T cells, obtained by neuraminidase-treated sheep erythrocyte (E-N) rosetting, and enriched mononuclear cells were cryopreserved using a programmed freezing procedure. A significant decrease of the TM and TG cells was found whereas the proportion of T cells and their subsets determined by monoclonal antibodies seemed not to be influenced. The effectiveness of T-cell separation by E-N rosetting of frozen lymphocytes demonstrated no impairment of the E-receptor binding capacity of T cells. The PHA reactivity of separated T cells was maintained after cryopreservation; however, the spontaneous blastogenesis was reduced significantly. The selective loss of the TM and TG cells seemed to be dependent on the length of the phase transition time; over 90 sec the capacity of the expression of Fc receptors was profoundly affected. Neither an additional 20 hr incubation after hypotonic shock prior to cryopreservation nor incubation after thawing could repair this function of T cells. The data suggest irreversible damage of the Fc receptor expression capacity on the cell membrane as a result of a disturbance of metabolic pathways rather than a preferentially greater sensitivity of these cells to cryopreservation.