Normal mouse serum (NMS) devoid of colony-stimulating factor (CSF) was found to enhance the interleukin 3 (IL-3)-driven colony formation of bone marrow in vitro. Inclusion of NMS in bone marrow colony-forming assays resulted in greatly increased numbers of colonies and clusters following seven days incubation; however, incubation of bone marrow with NMS before the colony-forming assay had no effect on resultant colony number. The levels of serum-enhancing activity (SEA) did not appear to vary significantly with age and in part was species restricted, in that human and guinea pig serum did not enhance mouse bone marrow colony formation. Conversely, NMS had no effect on human bone marrow colony formation. Levels of SEA were found to vary between strains, as did the degree to which bone marrow from various strains was enhanced by the serum. Serum fractionation studies indicated three active fractions with molecular weights of 800–900 Kd, 60–70 Kd, and 20- 30 Kd. The fraction at 800–900 Kd inhibited colony formation at high concentrations and enhanced colony formation on dilution, whereas the two other active fractions contained enhancing activity at all concentrations tested. These results would indicate that normal serum can play a greater role in colony-forming assays than nutritional supplements. The relationship of the SEA factors to other factors that have been reported to modulate bone marrow colony formation is discussed.