Murine bone marrow cells were separated on discontinuous Percoll gradients and assayed for their ability to give rise to megakaryocyte colonies. Ninety-one percent of the megakaryocyte progenitors (CFU-M) sedimented at densities between 1.070 and 1.080 g/mL. Six percent of CFU-M were found at densities between 1.060 and 1.070 g/mL, 2% between 1.050 and 1.060 g/mL, whereas less than 1% had a density either lower than 1.050 g/mL or higher than 1.080 g/mL. The number of doublings and endomitoses achieved by progenitors of density classes higher than 1.050 g/mL were similar. However, colonies derived from CFU-M of densities less than 1.050 g/mL (LD-CFU-M) had a higher probability of polyploidization and a lower probability of cell division in vitro. The inverse correlation found between the number of cells per colony and their DNA content was invariate regardless of the density class of the progenitors. The heterogeneity of the ploidy of cells within colonies increased continuously with increasing cell numbers per colony. The study if a short-period exposure of LD-CFU-M to acute thrombocytopenia could modify the ploidy of their progeny, mice were given rabbit antimouse platelet serum while control animals received normal rabbit serum. Twenty-four hours after injection, marrow was cultured. After a five-day culture period, no change in the number of colonies, doublings, ploidy, and heterogeneity of ploidy were observed between control and thrombocytopenic animals. The data suggests that LD-CFU-M are a distinct category of CFU-M, perhaps more mature than the common CFU-M.