The in vitro response to erythropoietin of bone marrow from nine patients with bone marrow failure were studied. Two types of patients were observed, those in which the marrow was responsive to erythropoietin and those which were nonresponsive. Ferrokinetic data corresponded well with the response to erythropoietin in vitro. In the nonresponsive group, a recovery of sensitivity to erythropoietin was observed when the patients improved clinically. The nature of the bone marrow failure was discussed in relation to erythropoietin and stem cells.