Reductions in the hemoglobin (Hb) mass are followed by increases in the level of plasma erythropoietin (EPO) concentrations which are directly proportional to the level of the hormone required to bring the hemoglobin values back to normalcy. Hypoplastic anemias (AHC) on the other hand, are accompanied by significantly larger increases in the erythropoietin concentration. In this study we have measured the relationship between the severity of the anemia and the levels of erythropoietin elimination in the urine patients (EpoU) under treatment with cytostatic drugs. Simultaneously other parameters used as indicators of bone marrow activity were determined. These observations suggest that the levels of erythropoietin depends not only by the tissue oxygen availability but by others factors related to the marrow cellularity.