AbstractSince the development of the first polyclonal antiserum to human milk fat globule (HMFG) membranes (1), this immunogen has been used by numerous workers to produce polyclonal and monoclonal antibodies. These reagents have been extremely useful in identifying markers of breast epithelial differentiation and have had a number of clinical applications. In addition, monoclonal antibodies to HMFG and to many tumour-associated antigens have brought into focus a group of highly immunogenic glycoproteins which have the characteristics of mucins and are expressed by many carcinomas. In this review we will discuss mainly work which has been done with the human milk fat globule, but it should be borne in mind that the same membranes and their components have been extensively studied in rodents and in the cow.