Transgenic mice carrying a fusion gene in which the mouse immunoglobulin enhancer has been inserted into an otherwise normal human c-myc gene develop a narrow spectrum of pre-B-cell lymphomas. Tumor occurrence is correlated with expression of the transgene in organs in which large numbers of pre-B cells predominate. These tumors, which arise stochastically, are virtually all lymphoblastic lymphomas of the pre-B-cell type. Evidently the isolated enhancer targets oncogene expression and tumorigenesis to the early B-cell population in preference to more mature B-cell populations. The transgene also confers enhanced in vitro growth properties on nontransformed pre-B cells as observed in bone marrow cultures derived from transgenic animals. These cultured cells represent a population in which the activating function of c-myc can be uncoupled from secondary oncogenic events occurring in vivo.