When the tetrazolium salt N-BT is used as indicator in the histochemical demonstration of dehydrogenase activity two differently coloured reduction products may be formed. Complete reduction yields blue granules (diformazans), while incomplete reduction leads to the production of rose-violet formazan compounds called half-formazans. The incidence of half-formazans in a material comprising 200 biopsy specimens of human breast tissue has been investigated. The half-formazans were seen in 75 per cent of the biopsy specimens from cases of carcinoma. In fibroadenomatosis 16 per cent of the biopsies showed half-formazans. The incidence was seen to increase appreciably with increasing degree of tumour anaylasia: (Grade I: 60 per cent, grade II: 72 per cent, and grade III: 89 per cent). The author considers that the half-formazan production is a direct consequence of alterations of the tumour-cell enzyme activity.