An elevated level of tumor necrosis factor-alpha in a cultural system of bone marrow erythroblastic islands was found. It was associated with the suppressed formation of erythroblastic islands de repeto in 24-hr cultures. In 48-hr cultures, an increased level of erythropoietin and intensive formation of erythroblastic islands de repeto and de novo was found. In 72-hr cultures, further elevation of TNF-alpha was accompanied by a decrease of erythroblastic islands in culture.