1. An assay for erythropoietic factor is described based on reticulocytosis in normal, adult mice. The advantages of the method are the short time required (4 days), the small amount of test material needed and the facility with which many tests may be run simultaneously.

2. For the measurement of uptake of Fe59 a procedure is described which allows counting of whole dried blood with an end-window Geiger-Müller tube.

3. A procedure is described for preparing highly active erythropoietic filtrates from the boiled plasma of rabbits made severely anemic with phenylhydrazine.

4. Evidence is presented that the ineffectiveness of whole plasma (anemic) on the one hand, and the effectiveness of the filtrate after the plasma is boiled on the other hand, is associated with respective presence and absence of antibody-provoking antigens.

5. A fractionation procedure is described whereby a fraction was obtained which, on the basis of amount of protein, is much more potent than unfractionated plasma filtrate, by tests based on reticulocytosis in mice, uptake of Fe59 in starved rats and polycythemia in normal rats. When this material was subjected to zone electrophoresis two-thirds of its protein and all the activity was found confined to one band. It is noteworthy that the material which gave the positive responses to the three tests has so far remained together in one fraction.

6. Some physical and chemical properties of erythropoietically active plasma fractions are described. So far erythropoietic activity has been inseparable from protein.

7. A number of hormones, vitamins and other growth-promoting materials were tested for erythropoietic activity and were found to be negative by the tests employed.

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