Conditioned media prepared using human placenta, spleen, bone marrow and peripheral blood leucocytes revealed a common pattern of two distinct species of colony-stimulating factors (CSF) separable by gel filtration. The peak of greatest activity, active against both human and mouse marrow, had an apparent molecular weight (MW) of 24,000-28,000 Daltons. A peak of low activity detected only against mouse marrow had an apparent MW of approximately 150,000 Daltons. The type of progenitor cells stimulated by the crude medium, by the low MW CSF species, and by the high MW species were similar for the four conditioned media despite their different origins. No difference was found in either the MW of the human active CSF present or the type of progenitor cells stimulated by media conditioned with cells of leukemic origin.