To asses the hematopoietic potential of fetal mesentery, we analyzed mesentery, liver, spleen and thymus from fetuses between 53 and 12S days gestation for cells expressing surface markers associated with hematopoiesis. The fetal mesentery contained populations of cells expressing CD4S, CD10, CD2S, CD38, CDS, CD2, CD69, c-ldt and HLA-DR, portions of which coexpressed CD34. Similar populations were detected in the liver, but the frequency in mesentery was 2-20 fold higher suggesting that fetal mesentery may have a greater hematopoietic potential than fetal liver. In addition, significant portions of cells (14-75%) from fetal mesentery expressed Thy-1 (CDw90) at high as well as at low to intermediate levels throughout the gestational period analyzed. The proportion of Thy-1cells in the mesentery coexpressing CD34 generally increased during gestation suggesting a developmental change in this population of cells. The spleen also contained Thy-1ceflj but very few coexpressed CD34 while the liver contained less than 3% Thy-1cells. Cells expressing low levels of Thy-T were present in the thymus at aD times tested. However, at early times in gestation, Thy-luCD34cells were found in both mesentery and thymus suggesting that similar populations exist in the thymus and mesentery during early fetal development.