It was shown that joint cultivation of supernatants with bone marrow erythroblastic islands in liquid culture systems resulted in stimulatory or inhibitory effects on erythropoiesis. Influence of supernatants on erythropoiesis depended on the culture age, erythrocyte production in these cultures and duration of joint cultivation with bone marrow islands. Supernatants of 24 h cultures induced formation of erythroblastic islands de repeto and de novo in cultures. Supernatants of 48 h cultures caused stimulation of erythropoiesis (to the end of 24 h period ofjoint cultivation) and processes of maturation in erythroblastic islands (at the 48th of joint cultivation). Supernatants of 72 h cultures inhibited formation of erythroblastic islands de novo and de repeto, but stimulated the processes of their maturation.