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.