Abstract
There is currently much speculation over the precise nature of lobular carcinoma in-situ of the breast and its significance. Histochemical study shows that it is rich in sialomucin. This has a characteristic intracellular distribution which distinghishes lobular carcinoma in-situ from cancerization of lobules. The same histochemical features are seen in the infiltrative phase of the tumour. These findings invalidate the concept that it is a myoepithelial-cell tumour. Practical applications of the distinctive pattern of mucin secretion are discussed.