A series of human multinucleate giant cells (MGCs) of the endocytotic type were studied using enzyme histochemical methods for dehydrogenases, glycosidases, phosphatases, and peptidases. Several enzyme patterns were found. The subgroup of MGCs associated with inflammatory granulomatous processes (sarcoidosis, granulomatous myositis, familial granulomatosis, lymphogranuloma, granulomatous cholangitis) was characterized by high activities of nonspecific esterase (NE) and tartrate-sensitive acid phosphatase (AcPase-Ts). There was no detectable activity of peptidases or tartrate-resistant isoenzyme of acid phosphatase (AcPase-Tr). This enzyme equipment was indistinguishable from that in mononuclear precursors in the granulomas. The other MGCs of the series displayed enzyme patterns substantially different from their monocytic precursors (blood monocytes and Langerhans cells). The subgroup of foreign body associated MGCs (resorption of fat, keratin, and suture material) was characterized by high activities of NE, AcPase-Tr, and greatly variable activities of both peptidases studied. The latter lacked predilection for certain subcellular regions. The subgroup of osteoclasts and so-called giant cell tumours (osteoclastoma, giant cell tumour of soft parts, giant cell epulis of peripheral, and central types) displayed very low activity of NE, high activity of AcPase-Tr, and strong activities of peptidases. The latter were localized near the surface membrane of the polykarya. MGCs in histiocytosis X (HX) differed from the previous group by higher values of NE in average. All MGC types had common denominator in the absence of alkaline phosphatase activity, on average intense dehydrogenase activities, mostly low beta-glucuronidase and highly variable alpha-mannosidase activities. The enzyme pattern heterogeneity is discussed with regard to the phenomenon of enzyme induction and depression occurring in course of polykaryon production. The variability of phenomenon may reflect reactive adaptation to varying functional demands imposed on MGCs under different conditions.