Objective. To determine the effect of fucoidan from Fucus evanes-cens on the dynamic processes of proliferation and apoptosis of human peripheral blood lymphocytes. Methods. Peripheral blood mononuclear cells (PBMCs) were prepared by Ficoll-Hypaque density gradient centrifugation and cultured in 96-well flat-bottom culture plates in complete were medium RPMI 1640. PBMCs were either unstimulated or stimulated in vitro with the mitogen phytohemagglutinin (final concentration 1 and 10 μg/ml), fucoidan (final concentration 1, 10, 100 μg/ml). At the end of the incubation, the number of apoptotic cells was examined by a cytofluorimetric method of measuring hypodiploid DNA stained with propidium iodide. Cell proliferation was measured PBMCs by [3H] thymidine incorporation into DNA. Results. Fucoidan from F. evanescens influenced the ability of PBMCs to blast transformation depending on the degree of proliferative cell potential: at the initially low level of spontaneous proliferation, the polysaccharide showed a stimulating effect, at the high level of spontaneous proliferation had an inhibitory effect. The direction of the effect of fucoidan also differed depending on the initial state of the cells (rest or activation) and the dose of the polysaccharide. Conclusions. The outcome of the activation of T-lymphocytes by mitogen and fucoidan can be both proliferation and apoptosis. The increase in the number of apoptotic cells upon introduction of fucoidan into the culture of lymphocytes activated by phytohemagglutinin is due to the increased sensitivity of activated cells to the induction of apoptosis at the height of proliferation and may be a reflection of the summation of mitogenic signals of fucoidan and phytohemagglutinin.