Three activated cellular raf genes have been detected by transfection of NIH 3T3 cells with human tumor DNAs. Blot hybridization analysis indicated that all three transforming raf genes had recombined with non-raf sequences in the vicinity of raf exon 7-intron 7, resulting in the deletion of about 40% of the normal coding sequence from the raf amino terminus. By cloning sequences upstream of the truncated raf loci we have shown that the rearrangements involve the fusion of three different 5' non-raf human sequences to the human raf gene. No rearrangements could be detected in the raf loci of the three original human tumor DNAs, suggesting that the raf genes were activated by DNA rearrangements occurring during transfection. Significant overexpression of raf mRNA was not evident in two of the three transformant lines, indicating that raf overexpression is not necessary and 5' truncation alone may be sufficient to activate the transforming potential of cellular raf genes.