Oxytocin produces numerous biochemical effects after binding to its membrane receptors, including the modulation of mitotic activity of various cell lines. The objective of this investigation was to evaluate the effects of oxytocin on mitotic activity of cultured human lymphocytes. We analyzed mitotic and proliferation indices, as well as cytokinesis block proliferation index in phytohaemagglutinin-activated human peripheral blood lymphocytes. Seven experimental concentrations of oxytocin were used in a range from 0.001 IU/ml to 0.1 IU/ml. Human recombinant insulin and N-methyl-N-nitro-N-nitrosoguanidine (MNNG) were used as positive controls. Mitotic index was not significantly influenced by oxytocin treatment. In addition, proliferation index (PI) and cytokinesis block proliferation index (CBPI) as parameters of cell cycle kinetics were not significantly changed after oxytocin treatment. Therefore, oxytocin exhibited neither stimulating nor cytotoxic (or cytostatic) effects on cultured human lymphocytes.