An antibody, 21N, raised against a synthetic peptide from the predicted sequence of the c-erbB-2 protein has been used immunocytochemically in a retrospective study of formalin fixed paraffin embedded breast biopsies. Fourteen out of 103 infiltrating ductal carcinomas exhibited positive membrane staining. Fifty-four of these tumours had lymph node involvement of which nine contained stained cells. These were all cases where the primary tumour was positive. In this series there was no correlation between c-erbB-2 overexpression and lymph node status. In five of the positive cases studied there was an associated in situ component which was also positively stained. Ten out of 24 pure intraduct carcinomas showed membrane staining, but none of the 149 benign conditions studied, which included 22 radial scars and 13 cases of atypical ductal proliferation, demonstrated the pattern of staining associated with overexpression. It is concluded that the c-erbB-2 protein is overexpressed in a minority (approximately 14%) of infiltrating ductal carcinomas and only in cells that are cytologically malignant. Overexpression of c-erbB-2 is considered in relation to pathogenesis.