We found an activated Kirsten (Ki)-ras gene in the MDA-MB-134 breast carcinoma cell line by transfection of NIH3T3 cells. Oligonucleotide hybridization demonstrated that this cell line carries a single G to C point mutation at position 12 leading to a glycine-arginine substitution. However, only a fraction of the cell population seems to contain this Ki-ras mutation. Since mutations can occur in cell lines during in vitro culture, we searched in breast carcinoma samples for the presence of single mutations at codon 12, but also for the presence of the double mutation previously found in the H-466B breast carcinoma cell line. Using polymerase chain reaction (PCR), we detected one primary tumour carrying a single mutation at codon 12. No double mutation was found in any of the tumours. These results show that Ki-ras gene mutation could be involved in breast carcinogenesis, albeit at a low frequency.