The c-myb gene encodes a sequence specific transactivator that is required for fetal hematopoiesis, but its potential role in other tissues is less clear because of the early fetal demise of mice with targeted deletions of the c-myb gene and incomplete of knowledge about c-myb's expression pattern. In the hematopoietic system, c-Myb protein acts on target genes whose expression is restricted to individual lineages, despite Myb's presence and role in multiple immature lineages. This suggests that c-Myb actions within different cell type-specific contexts are strongly affected by combinatorial interactions. To consider the possibility of similar c-Myb actions could extend into non-hematopoietic systems in other cell and tissue compartments, we characterized c-myb expression in developing and adult mice using in situ hybridization and correlated this with stage-specific differentiation and mitotic activity. Diverse tissues exhibited strong c-myb expression during development, notably tooth buds, the thyroid primordium, developing trachea and proximal branching airway epithelium, hair follicles, hematopoietic cells, and gastrointestinal crypt epithelial cells. The latter three of these all maintained high expression into adulthood, but with characteristic restriction to immature cell lineages prior to their terminal differentiation. In all sites, during fetal and adult stages, loss of c-Myb expression correlated strikingly with the initiation of terminal differentiation, but not the loss of mitotic activity. Based on these data, we hypothesize that c-Myb's function during cellular differentiation is both an activator of immature gene expression and a suppressor of terminal differentiation in diverse lineages.