Granulocyte-macrophage colony formation by C57BL bone marrow cells was initiated in agar cultures either by the granulocyte-macrophage stimulus, GM-CSF, or by the predominantly macrophage stimulus, M-CSF. After 24 hours, paired daughter cells of granulocyte-macrophage colony-forming cells (GM-CFC) were separated by micromanipulation and one cultured in GM-CSF, the other in M-CSF. From the differentiation pattern of the resulting colonies, irreversible commitment of some cells occurred during the first 24 hours and completion of the first cell division. A similar result was obtained using granddaughter cells present after 24 hours of incubation. However, when intact developing day 2 and day 3 clones were cross-transferred to GM-CSF or M-CSF recipient cultures, irreversible commitment was more obvious. Most M-CSF-initiated clones exhibited irreversible commitment to macrophage formation in GM-CSF cultures and a high proportion of GM-CSF-initiated clones continued to produce granulocyte progeny after transfer to M-CSF. The results indicated that GM-CSF and M-CSF can irreversibly commit the progeny of GM-CFC respectively to granulocyte or macrophage production. While for women GM-CFC this occurs within 24 hours and one cell division, for many cells, the process is slower and requires an incubation period of up to 48 hours and/or several cell divisions. Calculations from the data indicated that two-thirds of GM-CFC in adult C57BL marrow are biresponsive and respond to stimulation both by GM-CSF and M-CSF.