[en] Spleen cells from radiated mice (XS), but not from normal mice (NS), when combined with mouse marrow cells in vitro, had two effects on hemoglobin synthesis: (1) increased baseline, and (2) augmented erythropoietin-induced, hemoglobin formation. Spleen cells from normal and radiated rats caused increased hemoglobin synthesis by rat marrow cells in a dose-dependent manner. In addition, lung and thymus cells from radiated rats, but not from normal rats, caused increased hemoglobin synthesis by rat marrow cells. Rat NS augmented hemoglobin formation by mouse marrow, but neither mouse NS nor mouse XS had an effect on rat marrow. Medium from rat NS cultures stimulated hemoglobin synthesis when added to rat marrow cells. The increases in baseline, and in the response to erythropoietin, were dependent on the amount of ''conditioned medium.'' The substance in the medium responsible for these effects was not erythropoietin