The ability of small acetylcholinesterase-positive (SACHE) cells to incorporate tritiated thymidine (3H-TdR) was studied in the bone marrow of mice made acutely thrombocytopenic by injection of guinea pig antimouse platelet serum (APS) and in control mice injected with normal guinea pig serum (NS). 3H-TdR was administered in vivo, and bone marrow was collected at various time points thereafter. The initial labeling index (LI) for both groups was about 30%. After four hours, the LI increased and reached peak values at 48 hours, but was lower in thrombocytopenic animals. The lower peak LI in APS-treated animals may be the result of both a faster rate of influx of unlabeled cells into the acetylcholinesterase (AChE) positive cell compartment and efflux of more heavily labeled, and probably polyploid, SACHE cells into the compartment of recognizable megakaryocytes. In both groups the increasing LI occurred concomitantly with a decreasing mean grain count. This may indicate cellular division by some fraction of SACHE cells. Heterogeneity in nuclear morphology was also demonstrated, and the frequency of individual morphologies was altered in APS-treated animals. In addition, cells that incorporated 3H-TdR were larger than those that did not. This group of small cells appears to represent a pivotal point in megakaryocytopoiesis in which the switch from mitosis to endomitosis is occurring.