The long-lasting expression of an alpha-subunit of interleukin-3 receptor (IL-2Ralpha) was found to accompany the PHA-induced proliferation of human blood lymphocytes (HBL), so that to the end of the second day of mitogenic stimulation only, the large blasts may express the high affinity alphabetagamma(c) receptor for IL-2. With the selective pharmacological drugs to JAK (WHI-P131) and Src (PP2) it is shown that the non-receptor tyrosine kinases are involved in the surface CD25 expression. It is revealed that the PP-2-inhibitable expression of CD25 is timely associated with the initial stage of T cell activation, whereas WHI-P131-inhibitable expression was present during the whole G0/G1/S transition. These data indicate that at the early, antigen-dependent stage the expression of IL-2Ralpha is induced via Src-dependent signaling pathway, and prolonged increase in IL-2Ralpha expression is regulated by IL-2/IL-2 receptor interaction via JAK-dependent signaling pathway.