Histochemical demonstration of 5'-nucleotidase activity in inflammatory muscle disease. Using a histochemical method, 5'-nucleotidase activity was investigated in 80 muscle biopsy specimens, including specimens from eight patients with muscular dystrophy, 18 with nonspecific type II fiber atrophy, 15 with polymyositis, and 29 histologically normal controls. An interstitial reaction for 5'-nucleotidase was associated with an inflammatory infiltrate in 19 of 21 positive cases. Of the 15 cases of polymyositis, 14 showed an extensive interstitial reaction surrounding most of the myofibers and extending well away from the areas infiltrated by inflammatory cells. The extensive nature of this reaction makes 5'-nucleotidase activity a useful adjuvant in the diagnosis of inflammatory muscle disease. 