To determine if erythropoietin influences megakaryocytopoiesis, the purified recombinant human hormone (rEpo) was added to serum-free liquid cultures of murine marrow. A dose-related increment in acetylcholinesterase (AchE) production was observed. To assess if increments in this relatively megakaryocyte-specific enzyme marker were mediated by a direct hormone-megakaryocyte interaction rather than via an accessory cell population, rEpo was added to cultures of isolated single megakaryocytes. A significant, dose-related increase in cell size was noted in the presence of the hormone, accompanied by a high probability of an increase in cellular DNA content. The data show that rEpo does directly influence some aspects of megakaryocytic maturation, although the physiologic significance of this effect remains unknown.