A trial to determine the usefulness of recombinant human erythropoietin (rhEpo) as a mobilizing cytokine for patients with previously treated relapsed malignancies was performed. An initial peripheral stem cell apheresis collection was conducted during steady-state hematopoiesis for each patient to provide baseline data. rhEpo, 200 U/kg/day, was administered subcutaneously until the last apheresis procedure was completed. Immediately after the fourth daily dose of Epo, apheresis procedures were resumed and continued beyond five collections, when necessary, to accrue a total of 6.5 x 10(8) mononuclear cells (MNCs)/kg. Eight female and four male patients (median age = 44 years) were evaluated. Five to 14 (median = 8) apheresis procedures were performed for each patient. Toxicity attributable to Epo administration was negligible. Mobilization effects, as determined by an increase in the number of colony-forming units granulocyte/macrophage (CFU-GM) and burst-forming units-erythroid (BFU-E) in the apheresis products after Epo administration, were observed in all patients. Nine patients received high-dose chemotherapy and Epo-mobilized peripheral stem cell transplantation (PSCT). Beginning the day of the transplant, GM-CSF was administered until neutrophil recovery was satisfactory. The median time to recover 0.5 x 10(9)/L granulocytes was 16 days after PSCT. Epo appears to have mobilization properties. Further studies are needed to determine the clinical usefulness of Epo as a mobilizing cytokine. The addition of Epo to other mobilizing cytokines may provide increased effectiveness without adding toxicity.