The Copenhagen (Cop) rat carries a homozygous mammary carcinoma suppressor (MCS) gene that prevents both spontaneous and induced mammary cancer. Here we identify an additional rat strain, the Wistar-Kyoto (WKy), that is resistant to the development of mammary carcinomas. This rat strain is similar to Cop in having one homozygous copy of a mammary suppressor gene. The Cop MCS gene and the gene responsible for mammary cancer resistance in the WKy were found to be very closely linked genetically and thus are likely to be the same gene. It was found that, unlike the Cop strain, the WKy strain also carries multiple copies of a susceptibility gene(s). This is analogous to the susceptible Wistar-Furth strain, which carries three copies of dominant susceptibility genes. The MCS gene is epistatic in regard to these susceptibility genes. The availability of the WKy rat strain carrying MCS on a background independent of the Cop strain will aid both mechanistic studies and the molecular cloning of MCS.