The purpose of this report is to demonstrate the expression of very recently identified surface antigens on CD34+ and AC133+ bone marrow (BM) cells. Coexpression analysis of AC133 and defined antigens on CD34+ BM cells revealed that the majority of the CD164+, CD135+, CD117+, CD38low, CD33+, and CD71+ cells resides in the AC133+ population. In contrast, most of the CD10+ and CD19+ B cell progenitors and a fraction of the CD71high population are AC133−, indicating that CD34+AC133+ cells are enriched in primitive and myeloid progenitor cells, whereas CD34+AC133− cells mainly consist of B cell and late erythroid progenitors. This corresponds to the highly reduced percentage of CD10+ B cells and the absence of CD71high erythroid progenitors on AC133+ selected BM cells. A portion of 0.2-0.7% of the AC133+ selected cells do not coexpress CD34. These cells are very small and define a uniform CD71−, CD117−, CD10−, CD38low, CD135+, HLA-DRhigh, CD45+ population with unknown delineation. Four color analysis on CD34+CD38− BM cells revealed that virtually all of these primitive cells express AC133. Using an improved liposome-enhanced labeling technique for the staining of weakly expressed antigens, subsets of this population could be identified which express the angiopoietin receptors TIE (67.6%) and TEK (36.8%), the vascular endothelial growth factor receptors FLT1 (7%), FLT4 (3.2%), and KDR (10.4%), or the receptor tyrosine kinases HER-2 (15.4%) and FLT3 (CD135; 77.6%). Our results suggest that the CD34+CD38− population is heterogeneous with respect to the expression of the analyzed receptor tyrosine kinases.