In order to estimate the dependence of CD4 and CD8 antigen expression upon the cell activation, their presence on resting and activated human T cells was investigated. Peripheral blood lymphocytes of healthy donors derived from late (TEl) and early (TEe) E rosettes were used as resting and presumably in vivo activated T cell subsets, respectively. Expression of the above markers on T cells stimulated in vitro with PHA was also examined. It was found that both lymphocyte subsets contained similar percentage of CD4+ cells, nevertheless, TEe subset was partially enriched in CD8+ cells. PHA stimulation induced in TEe and TEl subsets the considerable increase of proportion of phenotypical opposing cells, CD8+ or CD4+, respectively. The above changes coincided with the time of the maximal DNA synthesis (the 72nd h of cell stimulation).