Transfection of rat mammary (Rama) 37 epithelial cells which yield non-metastasizing adenomas in syngeneic Wistar-Furth rats with a drug resistance plasmid containing both the neo gene and EJ-ras-1 (pSV2neo.ras) or with pSV2neo and a plasmid encoding the large T Antigen (pLT214) of polyoma virus yields drug-resistant transformants with a frequency of 10(-5). Representative transformants have been propagated in neo-selecting medium to yield various cell lines. The 7 lines transfected with pSV2neo.ras (EJ1 set) and the 10 lines co-transfected with pSV2neo and pLT214 (LT1 set) all produce tumours at subcutaneous (s.c.) sites with a shorter median latent period than tumours produced by the parental Rama 37 cells. In addition, the LT1 set of transformants yields a higher incidence of tumours than the Rama 37 cells. No metastases are produced when any of the oncogene transformants are inoculated s.c. into rats. However, when an EJ1 representative is inoculated intravenously (i.v.), tumour deposits are found in the lungs of the host animals. In contrast, other Rama 37 variants that metastasize from s.c. sites fail to produce any metastases when inoculated i.v. The oncogene transfectants contain integrated DNA that hybridizes to neo and to the requisite oncogenic DNAs; the pattern of hybridizing bands to the transfected genes and their expression as mRNA is complex, and is presented in detail.