Growth of human erythroid and erythroid-granulocytic colonies was investigated in a fibrinogen clot culture system. CFU-E, BFU-E, CFU-C and the mixed colonies were grown in cultures in medium conditioned by phytohaemagglutinin-stimulated human mononuclear cells, with small amounts of aplastic anaemia serum, but no exogeneous erythropoietin (Epo). The level of Epo in the culture was 0.015 U. The mixed colonies constituted 8-22% of the total colonies grown. The results indicate that high dose Epo is not required to grow BFU-E and the mixed colonies in cultures containing erythroid potentiating factor(s), and the frequency of bipotent haemopoietic stem cells in human bone marrow or peripheral blood may be higher than that reported previously.