The frequency of apocrine differentiation in breast carcinomas, assessed on purely morphologic grounds, is controversial. Apocrine differentiation in two cases of lobular carcinoma in situ (lobular neoplasia; LCIS) is reported for the first time. Using an immunohistochemical method for the detection of GCDFP-15, a protein present in apocrine epithelium and in the fluid of tension cysts of the breast, the apocrine differentiation in LCIS is confirmed. The histiocytoid variant of invasive lobular carcinoma is shown to be "apocrine" in nature, antigenically at least. The ultrastructural findings in one case of histiocytoid carcinoma are discussed in the context of an apparent discrepancy between the morphologic features of this tumor and the presence of an apocrine antigenic marker.