AbstractA 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 act