The effects of acute thrombocytopenia, produced by platelet antiserum (PAS), on both megakaryocyte colony-forming cells (Meg-CFC) and granulocyte-macrophage colony-forming cells (GM-CFC) were studied. During the 1-hr to 14-day period following acute thrombocytopenia (platelet counts < 5% of normal), bone marrow and splenic cells of C57BL/6J mice were obtained and cultured for 7 days in 0.3% agar. Numbers of GM and Meg colonies were determined. At no times were alterations in frequency of GM-CFC and Meg-CFC detected in femoral bone marrow. In contrast, GM-CFC in spleen were increased from 3 to 7 days after PAS and from 4 to 7 days after normal serum (NS). Increase in Meg- CFC in the spleen occurred from 3 to 5 days after PAS with a lesser, not significant increase after NS. Alterations in white blood cells and hematocrit values were not detected. Similar responses were observed in germ-free mice and after rechallenge of animals that had received PAS or NS 14 days previously. The delayed increase in Meg-CFC indicates that they are unlikely to be responsible for the altered megakaryopoiesis previously reported in bone marrow after acute thrombocytopenia and was not due to inhibition by PAS. The increase in GM-CFC may reflect stimulation of the reticuloendothelial system by heterologous proteins.