Transgenic mice bearing a c-myc oncogene subjugated to the lymphoid-specific immunoglobulin heavy chain enhancer (E mu) develop clonal B lymphoid malignancies, but most young E mu-myc mice lack malignant clones. Their prelymphomatous state has allowed us to examine how constitutive c-myc expression influences B cell development. We find that early stages are overrepresented, even before birth. Pre-B cells of polyclonal origin increase greatly, while B cells develop in reduced number. Both the pre-B and the B cells appear to be in an active state, since they are larger than normal and a greater fraction are in the cell cycle. Enforced myc expression has thus favored proliferation over maturation. Hence, a normal function of c-myc may be to regulate differentiation as well as to promote cell cycling.