To gain insight into the regulation of megakaryocyte precursors in vivo, (in vitro) megakaryocyte growth-promoting activity (Meg-GPA) was assayed in plasma of rats in which both marrow hypoplasia and thrombocytopenia had been induced by irradiation. Rats received whole body irradiation of 834 rad from a 137Cs source. Plasma was collected at intervals of hours to days, up through day 21 postirradiation, and was tested, at a concentration of 30%, for Meg-GPA on bone marrow cells cultured in 1.1% methylcellulose with 5 .times. 10-5 M 2-mercaptoethanol. With normal rat plasma, no megakaryocyte colonies (defined as .gtoreq. 4 megakaryocytes) were seen and only a few single megakaryocytes and clusters (defined as 2 or 3 megakaryocytes) were formed. Two peaks of plasma Meg-GPA were observed after irradiation. The 1st appeared at 12 h, before any decrease in marrow megakaryocyte concentration or platelet count. The 2nd occurred on days 10-14 after irradiation, after the nadir in megakaryocyte concentration and while platelet counts were at their lowest levels. A dose-response study of plasma concentration and megakaryocyte growth, using plasma collected 11 days postirradation, demonstrated that patterns of megakaryocyte growth were related to plasma concentration; formation of single megakaryocytes was optimal over a range of 20-30% plasma concentration, while cluster and colony formation were optimal at a plasma concentration of 30%. All forms of megakaryocyte growth were decreased with 40% plasma. There was a linear relationship between the number of bone marrow cells plated and growth of single cells, clusters and colonies using a concentration of 30% plasma collected 11 days after irradiation. Irradiation causes time-related increases in circulating megakaryocyte growth-promoting activity. The irradiated rat apparently is a good model for studying the relationships between Meg-GPA and megakaryocyte and platelet concentration in vivo.