T cell precursors in the chick embryo have been localized into the intraembryonic mesenchyme (IEM) and into the para-aortic region before the first wave of the thymic colonization on embryonic day (ED) 6,5-8. The cell surface markers of avian prethymic stem cells are not known. It is also not known whether these precursor cells are already committed to the T cell lineage before their thymic colonization. In 7-day-old chick embryos Ov+ cells were found in the para-aortic region. Also the endothelial cells of the embryonic dorsal aorta were positively stained. Ov antigen might represent a very primitive marker for precursor cells having the potentiality to differentiate both to haemopoietic and endothelial cells. Scattered CD45+ cells were observed in the same para-aortic area as in many haemopoietic areas in the loose embryonic mesenchymal tissues. CD8 alpha (MoAb 3-298) expressing haemopoietic cells were detected before thymic colonization on ED6. In flow cytometric analysis of IEM precursors Ov, CD45 and CD8 alpha expressing cells seemed to form distinct subsets suggesting heterogeneity of these haemopoietic cells.