Fresh-frozen sections of human breast tissue (210 biopsy specimens) are examined for activity of 5-nucleotidase in various benign and malignant pathological conditions. 5-nucleotidase is found mainly in the fibroblasts and produce a characteristic stroma-response. By nickel blocking of 5-nucleotidase it is possible to distinguish the 5-nucleotidase activity from that of the unspecific alkaline phosphatases. The technique is described. The two enzyme systems are both localized mainly in the fibroblasts, but at different sites.