Granulocyte and macrophage progenitor cells which clone in semi-solid agar cultures were characterized by separating murine bone marrow cells by equilibrium density centrifugation. These in vitro colony forming cells (CFU-c), when induced by stimuli in mouse lung-conditioned medium, were heterogeneous in their density properties. Differences were found in the dose responsiveness of 2 major subpopulations of these progenitor cells. One population (modal density 1.074 gcm-3) responded in an identical manner to stimuli present in mouse lung-conditioned medium and pregnant mouse uterus extract. A 2nd population (modal density 1.070 gcm-3) was markedly different in its dose responsiveness to mouse lung-conditioned medium, from the other subset (1.074 gcm-3). The cloning of the low density progenitor cells and the observed dose responsiveness correlated with an activity present in a stimulus in mouse lung-conditioned medium but was not identical to the stimulus in pregnant mouse uterus extract. No subpopulation gave exclusively granulocyte or macrophage colonies.