In order to elucidate the mechanism of determination and regulation of hemopoiesis, we have established a tissue culture system in which the mesodermal cells differentiate to hemopoietic cells in early embryo of Xenopus laevis. The production of hemoglobin was determined by Western blot analysis using the specific monoclonal antibody L5.41. The extensive erythropoiesis was observed in the explant of ventral mesoderm from tailbud embryo. The differentiation of erythrocytes in this ventral mesoderm explant was completely inhibited by the co-culture with dorsal tissue, including neural tube, notochord and somite mesoderm, but not by other regions of embryo. Also, we have found that the ventral mesoderm explant from early gastrula embryo, which alone was not able to differentiate into erythrocytes, can be stimulated for erythropoiesis in the combination with animal pole cells. These observations suggest that factors existing in the adjacent tissues positively and negatively regulate the development of erythroid precursor cells in the embryo. Furthermore, various growth factors and cytokines have been added to the ventral mesoderm explant from early gastrula embryo to find the possible factors which stimulate erythropoietic differentiation. We have demonstrated the stimulation of the globin expression in the ventral mesoderm by murine stem cell factor at the optimum dose of 10 ng/ml suggesting that stem cell factor functions physiologically in the process of erythropoiesis in embryo.