The characteristics with respect to light scatter and cell surface properties of megakaryocyte colony-forming cells (CFU-Meg), growing in serum-free agar cultures, have been determined. Mouse bone marrow cells were fractionated using a light-activated cell sorter (FACS) either on the basis of light scatter or after staining with a fluorescein isothiocyanate (FITC)-labeled lectin or monoclonal antibody. After staining and sorting, the recovery of CFU-Meg was about 50%. In the unsorted controls 29 +/- 2 CFU-Meg/10(5) cells were observed. The forward (FLS) and perpendicular (PLS) light scatter intensities of CFU-Meg were very similar to those of spleen colony-forming cells (CFU-S) and early progenitor cells. The average diameter of the CFU-Meg as determined by the FLS intensity profile was about 7.5 microns. Wheat germ agglutinin binding of CFU-Meg was high, indicating a high sialic acid content of the cell surface. Expression of the differentiation antigen Pgp-1 on the CFU-Meg as determined by indirect immunofluorescence with the rat monoclonal antibody I42/5.1 was moderate. Pgp-1 expression was lower than the Pgp-1 expression of granulocyte/macrophage progenitors (CFU-C) or granulocytes. It is concluded that the light scatter and cell surface properties of CFU-Meg are similar to those of CFU-S and early committed progenitors.