Mouse serum contains two erythroid progenitor-stimulating activities, one being highly increased in serum from anemic mice.

We previously observed that the addition of normal mouse serum to mouse bone marrow cell cultures increases the number of erythroid colonies arising from erythroid colony-forming units (CFU-E). In this paper, we show that the stimulating activity could be divided into two active fractions with 110 kd and 50 kd apparent molecular weight by gel filtration. The 50-kd fraction, in addition to its stimulating activity, increased the sensitivity of CFU-E to erythropoietin. The 110-kd activity was hardly detectable in normal mouse serum but was greatly increased in anemic mouse serum. This renders this activity of interest and suggests that it may play a regulatory role in mouse erythropoiesis.