The expression of the cell adhesion molecule CD15 (also known as Lewis x) by breast cancers and by adjacent normal and benign breast epithelium was investigated in a series of 98 tumours. Immunohistochemistry was performed on paraffin sections using the anti-CD15 monoclonal mouse IgM antibody Dako-M1. Some 35% of cancers expressed CD15, as did 45% of normal and 60% of hyperplasia. No association was observed between cancer cell staining, or any epithelial staining (cancer, benign and normal), and tumour size, histological grade, nodal status, age at diagnosis or the frequency of 'events' (recurrence or death). Chi-squared tests in each case were non-significant. The pattern of CD15 expression by breast cancer was frequently associated with the leading edge of invading tumour or with the outer edge of boli of carcinoma in situ, possibly suggesting a potential role in invasiveness, and with cancer cells trapped intravascularly, possibly suggesting a role in metastasis.