A murine monoclonal antibody (anti-MY7) has been developed that detects an antigen expressed by 6% of normal human bone marrow cells, including approximately 40% of myeloid colony-forming cells (CFU-C). The number of bone marrow cells and CFU-C expressing MY7 is significantly increased in regenerating bone marrow, but less than 5% of peripheral blood CFU-C express the MY7 antigen. Erythroid precursors are MY7 negative from peripheral blood and bone marrow. Thymidine suicide studies indicate that CFU-C in S-phase tend to be MY7 positive while CFU-C not in S-phase are MY7 negative. MY7 expression thus appears to identify a fraction of CFU-C that is actively proliferating.