1. Boiled plasma extracts from anemic rabbits are capable of stimulating erythropoiesis in normal rats as demonstrated by reticulocytosis, erythrocytosis, and increased marrow erythropoietic activity.
2. In the nonanemic rat recipient the increased erythropoiesis is manifested by the production of cells of decreased size without associated increase in the hemoglobin or hematocrit determinations. The cells probably have a greatly shortened life span.
3. The factor in anemic rabbit plasma is not destroyed by prolonged boiling and is not precipitated by perchloric acid. On the basis of these observations it would seem to be nonprotein in nature.
4. Further investigations concerning the role of the erythrocyte stimulating factor in normal and abnormal erythropoiesis should be facilitated by the use of nonprotein plasma extracts which may be administered to small laboratory animals.