Erythropoietin receptors induced by dimethyl sulfoxide exhibit positive cooperativity associated with an amplified biologic response. Erythropoietin triggers the differentiation of erythrocyte progenitors by binding to receptors on their plasma membrane. We report here that pretreatment of erythropoietin-responsive murine erythroleukemia cells with chemical inducers resulted in a striking increase in erythropoietin-specific hemoglobinization. This amplification of the erythropoietin biologic response was accompanied by the induction of a new population of high-density receptors (approximately 20,000 per cell) exhibiting marked positive cooperativity. Erythropoietin binding to new receptors displayed a convex upward Scatchard plot and a Hill coefficient (nH) of 6.75. Measurement of erythropoietin receptor mRNA demonstrated an initial decrease in receptor transcript followed by an approximately 2- to 3-fold increase after 24-48 hr. This increase in receptor message does not appear to account for the magnitude of the receptor up-regulation by dimethyl sulfoxide. We propose that this positive cooperativity reflects the interaction (clustering) of receptors, presumably through the formation of homooligomers or heterooligomers, and that this receptor interaction may amplify the erythropoietin signal transduction pathway. 