Bone marrow from 20 patients suffering from aplastic anemia were cultured with fetal muscle condition medium and in three different combinations: (1) Marrow from the patients cultured alone; (2) Coculture of marrow from patient with normal marrow; and (3) Culture of normal marrow with serum from patient. It was found that: (1) Nine patients had low colony formation when culture alone; no suppression in coculture and no inhibiter factor was found in serum from patient. Stem cells might be absent or defective in these patients. (2) Five patients had low colony when cultured alone; their marrow showed marked inhibitory effect on normal marrow CFU-C in coculture. and the latter was decreased by 40-100%. Suppressor cells might have caused the aplasia in these patients. (3) The sera from five patients suppressed CFU-C of normal marrow cells, colony counts being decreased by 55-77%. The cause of aplasia in these patients might be the presence of an humoral inhibiting factor. (4) In one patient no abnormal findings were found in all three different combinations of cultures. This patient might be suffering from a defective hematopoietic environment.