The expression of the c-erbB-2 proto-oncogene product was investigated immunohistochemically in 474 formalin-fixed and paraffin-embedded human breast tissue samples. The series included 32 benign and 26 hyperplastic lesions, 32 carcinomas in situ and 384 invasive breast carcinomas, 107 of which were less than 1 cm in diameter. Cytometric DNA assessments were performed on histopathologically or cytodiagnostically identified cell nuclei, using image analysis. C-erbB-2 immunoreactivity was not seen in normal parenchyma or in benign and hyperplastic lesions. Mammary carcinomas in situ were more frequently immunoreactive (59%) than invasive neoplasms (23%). Invasive tumours more than 1 cm in diameter immunoreacted more often (26%) than small invasive carcinomas (16%). C-erbB-2 expression in regional lymph node metastases was the same as in the corresponding primary tumours. Significant differences were observed between the c-erbB-2 expression in DNA diploid and aneuploid lesions; for carcinomas in situ the figures were 40% and 72%, respectively. Invasive carcinomas of DNA diploid type rarely showed c-erb-B-2 expression, irrespective of tumour size and nodal status (7-11%). DNA aneuploid tumours were more frequently immunoreactive with increasing levels during progression (32-41%). Our data indicate that genetically stable invasive mammary tumours seem rarely to express the c-erbB-2 protein, even during progression, whereas genetically unstable invasive neoplasms frequently show c-erbB-2 immunoreactivity which increases during tumour progression.