p53 mutations are frequent in human breast cancer. In order to understand the role of p53 in the context of the accumulation of mutations in breast cancer, a model of non transformed mammary cells was sought. The HC11 cells are immortalized, non transformed rodent mammary epithelial cells which synthesize milk proteins following stimulation with lactogenic hormones. p53 protein was readily detected in HC11 protein extracts with the PAb421 antibody. Two mutations were identified in the p53 cDNA from HC11 cells: a missense mutation at codon 138, substituting Trp for Cys, and a microdeletion, codon 123 to 130, of exon 5. The latter results from an intronic mutation of the splice acceptor site at the intron 4/exon 5 junction. The mutations affect separate p53 alleles, and no wt allele was found. Wt p53 was introduced into HC11 cells by means of a retroviral vector, under the control of a Cd(++)-inducible promoter. In the presence of CdSO4 a dramatic growth inhibition was observed. A temperature-sensitive mutant p53 gene was also transfected into HC11 cells. This resulted in a marked inhibition of cells growth at 32 degrees C, when the p53 is in the wt conformation, while no effect was observed at 37 degrees C, when the mutant conformation is predominant. wt p53-mediated inhibition of monolayer growth does not involve induction of programmed cell death and does not activate de novo synthesis of differentiation-specific milk proteins. We conclude that mutations in the p53 gene likely played a role in their immortalization. The HC11 cells provide a model for assessing the cooperative action of other mutations in mammary tumorigenesis.