A factor present in normal mouse serum stimulates late erythroid precursor proliferation

A factor present in normal mouse serum stimulates proliferation of late erythroid precursors grown in cultures in vitro. This factor was shown not to have a corrective effect on culture conditions by the following criteria: (a) CFU-E frequency in the absence of NMS was at least as great as published data for various mouse strains, (b) an inhibitory effect of endotoxin was ruled out, (c) sensitivity of erythroid precursors to erythropoietin was similar in the presence or absence of NMS, and (d) the number of colonies was linearly related to the cell dose. The enhancing effect of NMS was independent of hemin, transferrin, or dexamethasone, products all known to be stimulators of erythropoiesis. It was shown to be specific for CFU-E. We propose that this material be termed erythropoietic stimulating cofactor (ESCF).