Rhesus monkey embryonic stem (ES) cells undergo differentiation in vitro towere obtained with the latter system, suggesting that this variation may be species specific. We observed, however, that CD34 + and CD34 + CD38 -cells derived from ES cells expressed embryonic ε and ζ as well as α, β, and γ globin genes, whereas no expression of embryonic globins could be detected in the cell preparations from BM. Moreover, erythroblast-enriched CD34 -cells derived from 4-or 5-week ES cell differentiation cultures also expressed embryonic, fetal, and adult globin genes, with greater β gene expression, but otherwise were identical to those of the more primitive CD34 + cells derived from 2-week ES cultures. These latter observations may reflect the presence of heterogeneous cell populations within the cell fractions that were compared, or they may represent variability among ES-cell-derived hematopoietic stem cells.