Interferons are known to have modulatory effects on hemopoiesis. Human bone marrow mononuclear cells were employed to test the effects of human recombinant beta-interferon on myeloid and erythroid hemopoietic stem cell growth. Results demonstrated that 1,000 U/ml of beta-interferon significantly inhibited myeloid growth [colony-forming unit (CFU)-granulocyte macrophage] by 40-50%, whereas a higher concentration (10,000 U/ml) abolished CFU-granulocyte macrophage growth by 80-100%. The inhibitory effects of beta-interferon were partially reversible by increasing the concentrations of colony stimulating activity in the culture and could not be abrogated by addition of toxic oxygen radical scavengers such as superoxide dismutase and catalase. The inhibitory effect of interferon was found to be partially dependent on the presence of accessory cells, since less inhibition was seen using T-cell and monocyte depleted bone marrow cells. Lower concentrations of beta-interferon (10-100 units/ml) were without effect. In contrast to myeloid cells, the human erythroid progenitors (CFU-erythroid, burst-forming unit-erythroid) appear to be more sensitive to the inhibitory effects of beta-interferon. In this regard it was found that 100 U/ml of beta-interferon suppressed erythroid growth by 40-50%. These results suggest that human recombinant beta-interferon is capable of suppressing hemopoietic colony growth.