We here report a method for the characterization of carcinoma cells in cryosections of human breast tissue. Cryosections from 37 biopsies including carcinomas, fibrocystic disease specimens and reduction mammoplasties were incubated in an atmosphere of either pure nitrogen or 99.5% oxygen. The incubation medium contained neotetrazolium, phenazine methosulphate and the colloid stabilizer Polypep 5115. A marked overlap in formation of reaction product was found between carcinomas, benign tissue and normal breast tissue when sections were incubated in pure nitrogen. However, following 5 min of oxygen incubation no reaction was seen in normal breast tissue, whereas carcinoma cells consistently showed reaction. Moreover, a statistically significant difference was seen between the level of reaction in carcinoma cells and that of ductal epithelial cells in fibrocystic disease not containing apocrine metaplasia. Apocrine metaplasia, on the other hand, showed the highest activity recorded at all.