Summary.The cell surface molecule encoded by the proto‐oncogene c‐kit has recently been identified as the receptor for a growth factor variously termed stem cell factor (SCF), mast cell growth factor or steel factor. Using the c‐kit antibody 17F11 we analysed, in triple staining experiments, the surface molecule profile and scatter characteristics of c‐kit+CD34+human haemopoietic progenitor cells. In 10 normal bone marrow samples we found 19–51 of CD34+bone marrow progenitor cells to coexpress c‐kit. These c‐kit+CD34+bone marrow cells turned out to represent a phenotypically heterogeneous population. A considerable proportion coexpressed CD33 (52±23), and/or CD71 (62 ±26) antigens, marker molecules previously shown to be expressed by committed in vitro colony forming cells but not by their precursors. In line with a relatively differentiated phenotype c‐kit+CD34+cells also gave rise to on average higher forward and right‐angle light scattering signals. The proportions of CD38 and/or HLA‐D expressing cells were similar in the c‐kit+and in the c‐kit−subsets of CD34+progenitor cells. Coexpression of CD19 was found to be less frequent in the c‐kit+(4 ± 5) as compared to the c‐kit−(17 ± 14) fraction of CD34+cells. CD7+CD34+bone marrow cells were hardly detectable and their numbers too low to allow further sub
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