1. The thermostable erythropoietic factor in "anemic" rabbit plasma stimulates cellular division of marrow erythrocytic precursors in the normal rat producing erythrocytosis due to microcytes without evidence of augmented hemoglobin synthesis.
2. Study of erythrocyte osmotic behavior in these animals with a quantitative photocolorimetric method failed to demonstrate significant variation from the controls.
3. Osmotic fragility determinations by a direct cell enumeration technic revealed decreased resistance of the microcytes to lysis in hypotonic sodium chloride solutions.
4. It is concluded from these data that the erythrocytes in the normal rat injected with the heat stable plasma erythropoietic factor comprise two distinct cell types. They are microcytes with shortened survival times and normal erythrocytes.
5. The decreased viability of these microcytes is the apparent explanation for the rapid re-establishment of normal erythrocytic equilibrium after the "anemic" plasma extract injections are stopped.