Fractionation of conditioned medium derived from cultures of human peripheral blood leukocytes by ultrafiltration yielded material of low molecular weight, capable of stimulating the formation of granulopoietic colonies by human marrow cells in culture. The molecular weight of the active material was found to be less than that of vitamin B12 (1330 mol wt) on the basis of gel filtration on Sephadex G-25. The low molecular weight colony-stimulating activity (LMW-CSA) was trypsin-sensitive, and extractable into chloroform or diethyl ether. Unlike unfractionated leukocyte-conditioned medium, the LMW-CSA showed no colony-stimulating activity when tested on mouse, rather than human, marrow cell cultures.

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