Urinary extracts from idiopathic thrombocytopenic purpura (ITP) patients, aplastic anemia (AA) patients and normal subjects were investigated for their effects on in vivo platelet production, and both in vitro and in vivo megakaryocytopoiesis in rodents. Daily intraperitoneal injection of 1.2 absorbance units (AU, A278) of urinary protein for three consecutive days induced statistically significant increases in rat blood platelet numbers. This increase was observed for 1 of 4 ITP urinary extracts and for all 3 AA urinary extracts, and occurred 24 h after the final injection. In vitro levels of megakaryocyte colony-stimulating factor (Meg-CSF) in ITP urinary extracts were similar to those of normal urinary extracts, and were in dramatic contrast to the markedly elevated levels of Meg-CSF in extracts from AA urine. A single intraperitoneal injection of 0.5 AU of AA urinary protein induced a significant increase in spleen-derived megakaryocyte colony-forming cells (CFU-meg) 48 h past injection. In the group injected with ITP urinary extract, CFU-meg levels remained within normal limits. These results provide evidence that urinary extracts of ITP patients do not contain increased levels of Meg-CSF and a factor which directly stimulates in vivo CFU-meg production, and that the decrease in circulating platelet numbers that is characteristic of ITP patients is not a primary in vivo determinant in the elaboration of these factors.