Several clinical reports have suggested that prolactin (PRL) plays an important role in the pathogenesis of autoimmune diseases such as rheumatoid arthritis (RA) or systemic lupus erythematosus (SLE). We have investigated the influence of PRL on immune system, by evaluating the effects of PRL on the expression of CD69 and CD25 on human peripheral blood mononuclear cells (PBMCs). Human PBMCs obtained from healthy female volunteers were incubated with phytohemagglutinin (PHA) in the presence or absence of various concentrations of PRL. The expression of CD69 and CD25 was monitored using immunofluorescence staining and flow cytometry. PRL significantly enhanced the expression of CD69 and CD25 on activated PBMCs compared with that in the absence of PRL (p<0.05, paired t-test). Increasing doses of PRL enhanced the expression of CD69 up to 2 μg/ml and CD25 up to 1 μg/ml. The enhanced expression of CD69 was observed on CD8+ T lymphocytes but not on CD4+ T lymphocytes. Our data suggest that PRL can significantly enhance the expression of CD69 and CD25 molecule on human PBMCs when induced by PHA. However, PRL would have to be at optimal concentration in order to enhance their expression.