Owing to the contrasting observations in the field of interleukin(IL)-2 receptor research, the expression of IL-2 receptor chains was analysed on resting and anti-CD3 antibody (OKT3) activated CD4 and CD8 T cells by flow cytometry. Prior to the stimulation, 49% of CD4+ cells expressed IL-2Ralpha (CD25), whereas the expression of IL-2Rbeta (CD122) was very low (8%). The reverse was true for CD8 cells: 48% of them were positive for CD122, but only a fraction (10%) expressed CD25. Practically all lymphocytes expressed IL-2Rgamma (CD132). Interestingly, the unbalanced expression of IL-2Ralpha and -beta continued throughout the stimulation period of 2 days. In addition, the expression of CD45 isoforms in combination with the IL-2R chains and CD71 was followed during the activation of CD4+ T cells. Although CD45RA+/RO- CD4 cells were effectively activated, they retained their naive phenotype up to 2 days of stimulation. On the other hand, CD45RA+low/RO+low (Ddull) CD4+ cells shifted to the memory phenotype rapidly after being activated. However, by day 6 of stimulation the shift of both naive and Ddull cells to memory ones was obvious. The role of the IL-2 receptor in the activation of CD4 subpopulations is discussed.