SummaryTo study the action of corticosteroids on erythroid precursors, (burst forming unit-erythroid and colony forming unit-erythroid) in Diamond-Blackfan Syndrome (DBS), marrow from a newly diagnosed untreated infant was studiedin vitrowith prednisone and dexamethasone. This patient subsequently proved to be steroid responsive. Colony numbers increased linearly in an erythropoietin (EPO) dose response study. There was marked enhancement of colony numbers at all EPO doses after adding either prednisone (10-6M) or dexamethasone (10-9M) to the cultures. The data indicate that corticosteroids augment erythropoiesis at both early (BFU-E) and late (CFU-E) stages of development in DBS. In contrast, marrow from a second infant with DBS, clinically steroid resistant, failed to respond to steroidsin vitro.