Expression of both N-myc and c-myc is induced rapidly and dramatically in normal pre-B cells after stimulation with interleukin-7 (IL-7), a pre-B cell-specific growth factor. These IL-7-induced increases in N-myc and c-myc expression are mediated at the transcriptional level; for N-myc, a major portion of the induction results from release of an attenuation block between exons 1 and 2. Although c-myc expression has been shown to be regulated by many factors, these studies provide the first demonstration of N-myc regulation by a growth factor. Furthermore, mitogen-induced proliferation of more mature B-lineage cells is accompanied by induction of c-myc expression in the complete absence of N-myc expression. Thus, N-myc expression in normal B-lineage cells, as in transformed cells, is limited to cells that represent the precursor stages of this differentiation pathway. Together, these findings imply a specific function for N-myc in the early stages of B-cell development.