Two studies on breast cancer patients are described. Our aim was to examine whether the combined frequency of rare c-Ha-ras-1 alleles in cancer patients was raised. Firstly, the c-Ha-ras-1 locus in 56 breast cancer patients and 48 controls was examined for restriction fragment length polymorphism (RFLP) by Southern blot analysis of leukocyte DNA. Four predominant allelic fragments were found in both groups together with a variety of rare alleles. The 2 groups did not differ significantly in overall distribution of c-Ha-ras-1 alleles. Rare alleles combined were about as frequent in cases (7.1%) as in controls (6.3%). Secondly, 53 members of 3 families having a high incidence of breast cancer were c-Ha-ras-1 genotyped. None of 10 affected members was found to carry a rare c-Ha-ras-1 allele. The only c-Ha-ras-1 allele common to 11 affected members was a 6.8-kb allele which is found in 72% of the controls. Furthermore, this allele was found with equal frequency in affected and non-affected family members.