We have performed the molecular cloning of the non-ras transforming sequences previously detected in neoplastic Syrian hamster embryo fibroblasts initiated in vitro with 3-methylcholanthrene (MCA) (Notario et al., 1990). These sequences were isolated using cosmid-rescue techniques from a third-cycle NIH3T3 transformant obtained by sequential transfections of genomic DNA from MCA-initiated hamster fetal cells. Rescued (C-5) clones encompassed about 42.5 kbp of Syrian hamster genomic DNA containing hamster-specific repetitive elements (HRS). An internal 19 kbp BamHI fragment (B-1) was the only C-5 fragment which recognized specific transcripts in poly(A)+ RNA from hamster embryo cells. The same mRNA species were present in both normal and MCA-initiated neoplastic cells: a major transcript of about 2.5 kb, and other less abundant ones, ranging from approximately 2.0 kb to 5.0 kb. These mRNA species were detected consistently by each of several B-1 DNA subfragments located at positions spanning almost the entire B-1 length. The nucleotide sequence of some transcript-positive (S5P2 and S6) genomic B-1 fragments was determined. No significant homology exists between the nucleotide sequences of these B-1 subfragments and established DNA databases. Therefore, the C-5 cosmid clone contains novel genomic sequences. Transfection of C-5 DNA into mouse NIH3T3 cells resulted in the appearance of transformed foci (about five foci per microgram of DNA) within 25 days post-transfection, thus demonstrating the transforming activity of the C-5 clone, which was consequently renamed as the cph oncogene. Co-transfection of the cph oncogene with the human H-ras oncogene (T24), demonstrated a synergistic action between the two oncogenes in the transformation of murine fibroblasts.