Normal mouse serum contains two erythroid stimulating activities differing by their mode of action

We have previously shown that normal mouse serum contains two activities that increase the number of erythroid colonies obtained from murine bone marrow colony-forming units-erythroid (CFU-E) cultured in the presence of saturating Epo concentrations (0.2 U/ml). These activities could be separated by gel filtration and were referred to as peak I and peak II. We report here that peak I increases the number of erythroid colonies in cultures of adult bone marrow cells depleted of cells expressing Mac-1 antigen. On the contrary, peak II has no stimulating effect in these cultures unless Mac-1-positive cells are added to the culture. This synergistic effect is mediated by an activity released in the culture medium by Mac-1-positive cells.