OBJECTIVE To assess c-erbB-2 immunostaining in relation to morphological type of in situ and invasive breast carcinoma. METHODS Formalin fixed, wax embedded archival tissue was used. Invasive carcinomas comprised 50 infiltrating ductal (NOS); seven medullary, 10 tubular, 15 mucinous and 24 classic invasive lobular. In situ carcinomas comprised 48 ductal (DCIS) and 10 cases of lobular (LCIS). The antibodies used were pAB1 (polyclonal) which stains cell lines that over express the c-erbB-2 oncogene, and ICR 12 (monoclonal) which stains sections of breast carcinoma known to show c-erbB-2 amplification. RESULTS Immunostaining consistent with c-erbB-2 overexpression was found in 10 out of 50 cases of infiltrating ductal carcinoma (NOS), one of 24 infiltrating lobular carcinomas and one of seven medullary carcinomas only. Seventy per cent of ICR 12 positive cases of infiltrating ductal carcinoma also had extratumoral DCIS. Forty six per cent of pure DCIS lesions also showed strong membrane staining for c-erbB-2 protein, confined to large cell types. CONCLUSIONS Immunostaining for c-erb B-2 oncoprotein occurs mainly in large cell DCIS and infiltrating ductal carcinoma NOS, especially those with an extratumoral DCIS component. There is a low incidence in other types of breast cancer, including those associated with a better prognosis. Different biological mechanisms may be responsible for histologically distinct types of breast carcinoma.