Granulocyte and macrophage progenitor cells which clone in semi-solid agar cultures were characerized 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 found to be heterogeneous in their density properties. Differences were found in the dose responsiveness of two 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 second population (modal density 1.070 gcm-3) was found to be 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 was found to correlate 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, however, gave exclusively granulocyte or macrophage colonies.