An immunohistochemical study of c-erbB-2 expression was carried out on in situ (non-invasive) breast carcinoma, using antibody 21N, raised to the intracytoplasmic domain of the c-erbB-2 oncogene product. Strong membrane staining was observed in 44 out of 74 (59 per cent) cases of ductal carcinoma in situ (DCIS), but none of 48 lobular carcinoma in situ (LCIS) lesions. A detailed comparative morphological evaluation using several different parameters, including histological subtypes, was performed within the DCIS group. The results showed that there was a significant correlation between c-erbB-2 expression and the presence of large cell size, periductal lymphoid cell infiltration, marked nuclear pleomorphism, multinucleation, and a high mitotic rate. Of these, cell size appears to be the most important predictor of c-erbB-2 status, followed by the presence of periductal lymphoid cell infiltration. These results indicate, firstly, that LCIS and DCIS are biologically (as well as histologically) different and, secondly, that a subgroup of DCIS, which is associated with c-erbB-2 over-expression, exists and appears to have distinct histological features. The subgroup of DCIS cases which over-express c-erbB-2 may be a biologically definable category with prognostic importance. These results may therefore have relevance to breast screening programmes, but a larger study incorporating clinical data would be necessary to correlate these findings with clinical outcome.