Biologically active erythropoietin recovered in crude form from the urine of anemic patients and supplied by the Department of Health, Education and Welfare is being employed as a starting material from which to prepare highly purified erythropoietin. A variety of procedures, some well characterized and others quite new, that have been applied successfully to the purification of protein or other glycoprotein materials are being investigated for their value as possible steps in the recovery of this acidic glycoprotein hormone. Potencies of materials are measured by the plethoric mouse bioassay procedure and by radioimmunassay. Active material of differing potencies as it becomes available will be distributed or used for studies designed to give some insight into those processes through which erythropoietin controls red blood cell production.