A batch of erythropoietically active material made from the plasma of phenylhydrazine treated rabbits was distributed to seven laboratories in this country for the purpose of comparing assay methods. The results of the assays using 10 different methods or major variations are presented.

Most of the assay methods being employed were represented so that other laboratories should be able to compare their results to those obtained in this study.

The importance of obtaining some uniform basis for comparison of activity of erythrocyte stimulating factor between laboratories has been emphasized and a tentative unit has been proposed based on a net incorporation into rat red cells of 20 per cent of injected Fe59, using a standardized assay procedure.

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