Human placental conditioned medium (HPCM) contans colony-stimulating factors (CSFs) required for the growth in vitro of neutrophilic granulocyte-macrophage (GM) and eosinophilic (EO) progenitor cells from human bone marrow. Fractionation of CSFs in HPCM was achieved by manipulation of the elution conditions on a column of phenyl-Sepharose. After equilibration of the phenyl-Sepharose column at high ionic strength (1 M ammonium sulfate), all of the CSF bound; one species of GM-CSF (alpha) and all of the elutable EO-CSF were eluted from the column simply by reducing the salt concentration, whereas the second species of GM-CSF (beta) was free of EO-CSF and was eluted only by increasing the concentration of tehylene glycol in the elution buffer. The two GM-CSFs were functionally distinct. GM-CSF alpha preferentially stimulated colony formation by day 14 of culture, and there was a decreased proportion of neutrophil colonies and increased proportion of macrophage colonies as the strength of the stimulus was decreased; GM-CSF beta, on the other hand, preferentially stimulated colony formation by day 7 of culture, and the proportion of neutrophil colonies was high (average 80%) and independent of the concentration of GM-CSF beta. GM-CSF alpha and GM-CSF beta were indistinguishable on the basis of apparent molecular size on tel filtration columns (molecular weight 30,000), charge properties on isoelectric focusing beds (isoelectric point, 4.9), and were not related to each other as a sialoglycoprotein is related to its asialo form. Adherent cell removal of the target bone marrow cells (to remove colony-stimulating cells) suggested that both GM-CSFs acted directly rather than by stimulating the production of GM-CSF. Mixing and titration experiments indicated that the differences in functional specificities of the two GM-CSFs (and the lack of EO-CSF associated with GM-CSF beta) were not due to the presence of specific inhibitory molecules or lower absolute levels of CSF in one fraction relative to the other. These two species of GM-CSF should be useful in separately enumerating subpopulations of different GM-progenitor cells inhuman hemopoietic disorders.