When mouse bone marrow cells are seeded in agar cultures containing erythropoietin or pokeweed mitogen stimulated spleen cell conditioned medium plus erythropoietin, megakaryocytes are found mixed with erythroid cells in approximately 40% of the erythropoietic bursts that develop in the cultures. Chromosome spreads of C-metaphases in such “megaerythro bursts” were prepared and stained in situ with a modification of the C-banding technique. In cultures seeded with mixtures of male and female cells, metaphases from individual megaerythro bursts were shown to be either all male of all female but not both. Moreover, tetraploid C-metaphases of megakaryocytes were found to be of the same sex as diploid C-metaphases of erythroid cells in the same megaerythro burst. These results provide evidence that in the mouse, a bipotential progenitor cell exists that has the capacity to give rise to cells of both the megakaryocytic and the erythrocytic lines of differentiation.