BACKGROUND Erythropoietin, which is a hematopoietic growth factor, has been found to play a role in various physiologic processes within the body including testicular steroidogenesis and spermatogenesis. However, it is not known whether erythropoietin is also essential for the normal physiology of mature sperm cells. In this study, the effects of recombinant human erythropoietin beta (rEPO) on sperm motility were investigated. METHODS Samples of 37 volunteers (with total motile sperm count>5x10(6)/ml and a total motility of >50% according to WHO criteria) were collected by masturbation following a 3-5 days period of abstinence. After morphometric analysis before and just after washing, samples were either used as control or treated with rEPO at concentrations of 0.1, 1, 10 or 100 mIU/ml, respectively. Control and treated tubes were incubated for 4 h at 37 degrees C. RESULTS Total motility, total progressive motility, slow forward and nonmotile sperm counts of 1, 10 and 100 mIU/ml rEPO groups were significantly improved. This effect was dose independent. CONCLUSIONS No significant effect was found at 0.1 mIU/ml concentration. These results suggest that supplementation of media used for sperm preparation techniques with erythropoietin might be beneficial. Further studies are needed to clarify the mechanism of action of erythropoietin on mature sperm cells.