C-myc and c-Ha-ras oncoprotein expression was studied by immunohistochemistry and gene detection by in situ hybridization on serial frozen sections of 32 breast lesions (19 benign biopsies and 13 infiltrating carcinomas). C-myc protein was expressed in 15/19 benign and 12/13 malignant lesions; c-myc gene was detected in 17/19 benign and 13/13 malignant lesions. Although a higher proportion of benign biopsies (8/9) showed more than 50% of protein-positive cells than malignant specimens, this cannot predict the outcome of a lesion. Conversely, p21 ras protein was expressed only in 2/19 benign lesions and in most cases of grade I to III carcinomas. The c-Ha-ras gene was always detected in a small percentage of cells, in both benign and malignant lesions. The results obtained with atypical hyperplasia, a doubtful proliferating lesion, suggests that p21 c-Ha-ras protein expression is not restricted to breast carcinomas. Although Southern blot is commonly considered as a very sensitive technique for oncogene analysis, no amplification of c-myc and c-Ha-ras gene has been demonstrated either in benign or malignant lesions. The detection, on serial frozen sections, of proteins and DNA of c-myc and c-Ha-ras, showed a possible amplification of the c-myc and c-Ha-ras genes in various benign and malignant lesions.