Previous studies from this group have documented the secretion of erythropoietin by several isolates of erythroleukemia lines. A relatively rapid and simple purification scheme has been developed for erythropoietin and involved lectin chromatography, gel filtration, ion exchange chromatography, hydroxylapatite chromatography, and finally high pressure liquid chromatography. Polyacrylamide analysis indicates that this material is pure, but sequencing, now in progress, will prove or disprove its homogeneity. This will permit us a route to clone the erythropoietin gene which will open many doors in hemopoietic physiology. In following up on this discovery, we made a provocative observation which has far-reaching implication for cancer detection, therapy and, above all, prevention. We observed that leukemogenic tumor cells could be detected in specially designed hormone sensitivity assays long before leukemia was clinically apparent. The cells which grew in the presence (but not in the absence) of the hormone were shown to produce a fatal leukemia when inoculated back into animals. This sensitivity of the tumorigenic cells led us to test whether reduction of the hormone in vivo might prevent the leukemia from developing. In the majority of treated mice, the tumors disappeared and the animals have appeared healthy for more than one year. If treatment was given later in the disease, the animals did not complete remission but did survive 2-3 times longer than non-treated controls.