Ultrastructural immunocytochemistry was used to analyze cell surface distribution and intracellular localization of milk fat globule membrane antigen (MFGM-A) in cryosections from human breast carcinomas and benign breast biopsy specimens. The specimens were fixed in formaldehyde and frozen. Cryostat sections were cut at 15 micron, incubated with mouse monoclonal antibody to MFGM-A, and then with a peroxidase-conjugated goat anti-mouse antibody. After glutaraldehyde fixation, the sections were incubated with diaminobenzidine-H2O2 and further processed for electron microscopy. MFGM-A was specific for epithelial cells. MFGM-A staining was strictly confined to the apical surface membrane of normal ductal epithelium, never involving basolateral membranes below the tight junctions. In normal epithelial cells, MFGM-A was readily detected in cisternae of the endoplasmic reticulum (ER), but only to a lesser extent in Golgi complexes and presumptive secretory vesicles. In carcinoma cells, surface staining for MFGM-A was either distributed in a non-polarized manner on the entire cell surface or else was totally absent. In some carcinoma cells without surface-associated MFGM-A, very pronounced intracellular MFGM-A staining was seen in the ER, in the nuclear envelope, and in annulate lamellae. The observations on MFGM-A expression were supported by studies on a cell culture model system.