In the avian embryo, the wall of the aorta is a site where haemopoiesis occurs in large diffuse foci from day 3 to day 10. In contrast to haemopoiesis in other organs of the embryo, para-aortic haemopoiesis is sustained by stem cells, which emerge in situ. Previous studies have demonstrated that the para-aortic region from the day-4 chick embryo harbours committed myeloid progenitors and committed erythroid progenitors. The present paper reports the in vitro development of para-aortic progenitors with both myelomonocytic and erythroid potentialities. Three types of myelo-erythroid progenitors were observed, giving rise to erythroblasts and monocytes, to erythroblasts and granulocytes, or to erythroblasts, monocytes and granulocytes. Their frequency in the para-aortic cell suspension was 1 per 10,000 cells. In cell sorting experiments, they co-sorted with committed progenitors in the cell population that immunolabeled with the VI-A2 monoclonal antibody, which is specific for chicken haemopoietic cells. Cell sorting also demonstrated that these multipotential progenitors did not express the BEN cell surface molecule, in contrast to late myeloid progenitors. The BEN molecule belongs to the immunoglobulin superfamily and is expressed by haemopoietic progenitors from bone marrow, selective sets of neurons and epithelial cells from the bursa of Fabricius. The myelo-erythroid progenitors were enriched 4 times in the VI-A2-positive cell population, and 2 to 5 times in the BEN-negative population. These results represent the first in vitro demonstration of avian normal myelo-erythroid progneitors.