We investigated the in vivo effects of a crude extract from the urine of aplastic anemia patients (AA urinary extract) on erythroid precursor cells in the femoral bone marrow and spleens of normal adult mice. A single intraperitoneal injection of AA urinary extract induced a significant increase in the number of splenic erythroid burst-forming units (BFU-e) and erythroid colony-forming units (CFU-e) within 24 h after injection. We then injected pure recombinant erythropoietin (Epo) equivalent to the amount present in the urinary extract. This addition increased the number of splenic CFU-e by almost the same degree as the amount induced by the AA urinary extract 24 h after injection, but failed to elicit any change in the number of splenic BFU-e. In other studies, mice were injected with the same amount of lipopolysaccharide (LPS) and/or pure Epo as that present in the AA urinary extract. Experiments with Limulus amebocyte lysate-adsorbed (endotoxin-depleted) or nonadsorbed (endotoxin-containing) AA urinary extracts showed that endotoxin contamination interfered with the increase in numbers of marrow CFU-e and enhanced the increase in splenic CFU-e numbers induced by pure Epo or Epo activity in the AA urinary extract. The number of splenic BFU-e, however, was not affected by administration of LPS and/or Epo or by adsorbed endotoxin. These data suggest that AA urinary extract contains a stimulating activity for mouse splenic BFU-e, and that this activity is not attributable to the Epo activity or endotoxin contamination within the urinary extract.