The activation of closely related transcription factors STAT5alpha and STAT5beta is one of the key signalling events during interleukin-2-dependent stage of T-lymphocyte proliferation. Here, we investigate the activation of STAT5 and STAT3 family proteins on different stages of T-cell proliferative response to various mitogenic stimuli. The activation of STAT proteins was estimated according to their tyrosine phosphorylation. Isolated peripheral blood lymphocytes were stimulated by phytogemagglutinin (PHA), or by phorbol-12,13-dibutyrate (PDBu) with calcium ionophore--ionomycin, or by exogenous interleukin-2 (IL-2). After activation of T-cells by PHA as well as by PDBu with ionomycin we observed phosphorylation of STAT5 not earlier than in 5 h. The maximum phosphorylation (by 10 times and more, as compared to control resting lymphocytes) was detected following 24 h of mitogen stimulation. In the lymphocytes pretreated by PHA in submitogenic concentration, exogenous IL-2 induces a proliferative response. High phosphorylation level of STAT5 was determined 10 min after addition of IL-2 and kept for the next 24 h. In contrast to STAT5, some basal phosphorylation of STAT3 was found in resting and PHA-pretreated T-cells. The profile of phosphorylation STAT3 during mitogen-induced activation is more leveled. An immunosuppressant cyclosporine A taken at antiproliferative concentration decreased phosphorylation of both STAT5 and STAT3 by 1.5-2.0 times at early stages, as well as at late stages of activation. A correlation between alteration in tyrosine phosphorylation level of STAT5 and the expression of the high affinity IL-5 receptor was established. Taken together, we report that the increased STAT5 phosphorylation can serve as a marker of T-lymphocyte entrance into IL-2-dependent stage of proliferation after T-cell activation by different mitogens.