We have developed a technique for assessing bone viability by the histochemical demonstration of lactate dehydrogenase (LDH) activity in osteocytes. Fresh sawn and ground (75 microns) sections were prepared from femoral heads removed at operation from patients with osteoarthritis of the hip. The sections were decalcified overnight in cold 10% EDTA, pH 7.0. LDH activity was shown by the tetrazolium-formazan reaction with nitroblue tetrazolium as indicator and lithium lactate as substrate. Osteocytes were regarded as viable if their cytoplasm stained dark blue, indicating LDH activity; lacunae containing non-viable osteocytes could be identified by interference contrast illumination. Nearly all osteocytes were viable in the samples studied. Small trabecular fragments, such as could be obtained by needle biopsy, were also suitable for staining after grinding to approximately 50 microns. The method should have application both in research and in diagnosis of ischemic bone disease.