Transgenic mice bearing c-myc driven by the immunoglobulin heavy chain enhancer (E mu) initially exhibit a preneoplastic lymphoproliferative syndrome from which clonal pre-B or B lymphomas develop at random. To investigate whether this transition involves the activation of oncogenes capable of transforming fibroblasts, we transfected DNA from 14 E mu-myc lymphomas into NIH3T3 cells and tested the tumorigenicity of the transfectants in nude mice. By this assay and subsequent direct analysis of the lymphoma mRNA by cDNA cloning or the polymerase chain reaction, two independent E mu-myc lymphomas were shown to contain an N-ras or a K-ras oncogene mutated at codon 61. When incorporated into a recombinant retrovirus, the mutant N-ras allele could collaborate with myc to transform preneoplastic E mu-myc bone marrow pre-B cells. These results indicate that spontaneous mutation of ras genes is one pathway to lymphomagenesis in E mu-myc mice but that many of the lymphomas arise in response to changes that do not register in fibroblasts.