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Three distinct classes of epitopes on human CD164 have been identified. Two of these, recognized by the monoclonal antibodies 105A5 and 103B2/9E10, are the CD164 class I and class II functionally defined epitopes, which cooperate to regulate adhesion and proliferation of CD34⁺ cell subsets. In this article, we demonstrate that these 2 CD164 epitopes are expressed on CD34⁺ cells throughout ontogeny, in particular on CD34⁺ cell clusters associated with the ventral floor of the dorsal aorta in the developing embryo and on CD34⁺ hematopoietic precursor cells in fetal liver, cord blood, and adult bone marrow. While higher levels of expression of these CD164 epitopes occur on the more primitive AC133^hiCD34^hiCD38^lo/− cell population, they also occur on most cord blood Lin⁻CD34^lo/−CD38^lo/− cells, which are potential precursors for the AC133^hiCD34^hiCD38^lo/− subset. In direct contrast to these …