In murine erythroleukemia cells, the minor/major hemoglobin (Hb) ratio depends on the cell line and the inducing agent. To determine whether mouse minor hemoglobin is a “fetal” hemoglobin in vivo, globin chain composition and synthesis rates were determined in DBA/2 mice of various ages ranging from 14-day embryos to > 6-mo adults. Globin chains were separated by electrophoresis on polyacrylamide gels containing urea and Triton X-100. This method separates the embryonic (x,y,z) and the adult (alpha, beta ma, beta mi) globin chains. Fourteen day embryos had only 5%-10% adult globins but approximately 30% of the adult beta chains were beta mi. The % beta mi decreased with age and reached 20% in adult mice. Biosynthetic studies led to more pronounced differences: beta mi synthesis was 45% of total beta chain production in 14-day embryos and declined to 22% in adults. Thus beta minor/total beta globin synthesis declines during mouse ontogeny. This resembles qualitatively the human switch from fetal to adult hemoglobin and provides a murine model for studies of hemoglobin regulation.