Results of studies on the in vitro production and release of colony stimulating factor (CSF) by bone associated cells cultivated in the presence or absence of mouse sera are presented. Tissues from 3 strains of mice were equally effective in the production of CSF in vitro. Priming of the tissue donors by endotoxin [Escherichia coli] 4 h previously, or by irradiation 1, 7 or 15 days previously, significantly increased the CSF production. The addition of serum from normal mice to the culture medium had no effect on CSF production by tissue from normal mice, but regularly inhibited CSF production by bones from irradiated animals. The inhibition was not due to the toxic effect of the sera on the CSF producing cells and was more pronounced in the cultures of NMRI than in the C57BL tissues. Some inhibition was observed in cultures of tissue from endotoxin-primed NMRI mice. Extraction of the sera with ether removed the inhibitory material. The sera which inhibited the CSF production in vitro also inhibited the growth of granulocytic and macrophage colonies in soft agar cultures, but did not inhibit the growth and DNA synthesis of cultured [mouse fibroblast] L cells.