Cell cultures of 7,12-dimethylbenz[a]anthracene-induced rat erythroleukemia can be stimulated to synthesize hemoglobin when cultured in hypertonic media. During hypertonic treatment the intracellular osmotic conditions immediately readjust to those of the extracellular medium. None of the Friend virus-induced mouse erythroleukemia cell lines was inducible for differentiation with the same hypertonic culture conditions used for rat cells. Earliest commitment to erythroid terminal differentiation by hypertonic treatment of cells grown in either Eagle's or Joklik's medium occurs coincident with the commitment induced by hexamethylenebisacetamide. These results make it likely that induction of differentiation in rat erythroleukemia cells is a cell membrane-mediated event.