The effect of human recombinant erythropoietin (rEPO) on the reticulocyte count and the reticulocyte maturation distribution in rats was measured with an automatic reticulocyte analysis system (AURAS). The strongest reaction was found on the third day after a single application of rEPO with respect to reticulocyte count, maturation distribution, and the count of immature reticulocytes. At this time these parameters show a distinct dose dependency. The results indicate a possible basis for a simple rEPO bioassay using normocytemic rats.