Changes in the expression of H-2K antigen on murine hematopoietic progenitors (CFU-E, BFU-E, CFU-C and CFU-mix) were examined during murine development using the complement-dependent cytotoxicity assay. The results revealed that CFU-mix, BFU-E, and CFU-E had begun to express H-2K antigen as early as day 16 of gestation, and antigens increased sequentially thereafter. On the other hand, CFU-C expression began as late as days 12-15 after birth and the antigen increased rapidly until week 8 after birth. Expression of H-2K antigen on CFU-mix was higher than that on CFU-E and BFU-E, while that on CFU-C was lowest during the period between day 16 of gestation and days 12-15 after birth. The possibility of an indirect effect of antibodies on T cells was unlikely because thymocytes added to week-8 bone marrow cells after antibody treatment did not produce a significantly different effect on the culture.