Summary.Evidence is presented for an inhibitory action of the bone marrow adherent cell layer on erythropoiesis in a case of pure red cell aplasia. Cultures of the patient's bone marrow with autologous and normal plasma showed normal numbers of CFU‐C, but total absence of BFU‐E. There was no inhibition of BFU‐E when normal bone marrow was cultured with the patient's plasma. T‐cell depletion of the patient's bone marrow did not affect these results, but after removal of the adherent cells, normal numbers of BFU‐E were cultured. These observations suggest that the adherent cells, which are predominantly macrophages, were responsible for the inhibitory action on eryth