Characteristics of murine megakaryocytic colonies and their progenitor cells (CFU-m) were studied in vitro in agar gel. Colony growth required the presence of poke-weed-mitogen-stimulated spleen-conditioned medium. The number of colonies formed was linearly related to both the number of marrow cells plated and the amount of conditioned medium added. In addition, CFU-m were found in both the spleen and peripheral blood. Conditioned medium was also made without plasma, and this resulted in a cloning efficiency greater than that of conditioned medium prepared with plasma. The percentage of CFU-m in DNA synthesis was low (10%), as determined both in vivo and in vitro. Velocity sedimentation revealed that the majority of CFU-m sedimented at 4.3 mm/hr and had a tritiated thymidine (3H-TdR) suicide rate of 1.5 +/- 1.5%. A shoulder on the profile of CFU-m sedimented at approximately 6 mm/hr, with a suicide rate of 79 +/- 2%. Analysis of these data indicated that the majority of CFU-m were not in cycle or were in a long G1 period. The results suggest that CFU-m is a primitive progenitor, possibly closely related to murine splenic colony-forming units (CFU-s), analogous to erythroid bursts and granulocytic colony-forming units.