The present study describes a cytochemical approach to demonstrate human breast carcinoma cells in cryosections and in primary monolayer cultures from surgical biopsies. The material consisted of biopsies from 52 carcinomas and 29 benign lesions. Cryosections and cultures were incubated to demonstrate NADPH-neotetrazolium reductase in an atmosphere of 99.5% oxygen. Incubation time and section thickness were adjusted to accomplish the same level of reaction in cells of cryosections and corresponding cultures. Positive reaction was thus confined to epithelial elements and to the wall of some smaller blood vessels. More than one-half of the carcinoma cells showed moderate to strong reaction in cryosections from 29 of 52 carcinomas whereas no reaction was seen in ductules of normal appearance adjacent to these carcinoma cells. Positive reaction was seen in epithelial cell islets in primary cultures of 16 of the 40 carcinomas cultured. In cryosections from flbroadenomas and flbrocystic disease specimens only apocrine metaplasia consistently showed positive reactions compared with less than 10% of other ductular profiles in a given cryosection. This pattern of reaction was reflected in the derived primary cultures in which positive reaction was found in epithelial cell islets in only one of 19 biopsies cultured. The presence of human milk fat globule membrane antigen was used to demonstrate the epithelial nature of the cell islets seen in cultures of biopsies from both benign lesions and carcinomas. NADPH-neotetrazo-lium reductase positive islets from carcinoma biopsies were frequently aneuploid whereas most negative islets from carcinoma biopsies were diploid as were all islets from benign tissues.