Zinc is an important element in biology yet little is understood of its role in bone cell metabolism and function. This study examined the effects of zinc on osteoclast (OC) function in cultures derived from neonatal rats and in cocultures of OC and UMR 106-01 osteoblast-like cells (UMR/OC cocultures). Treatment with zinc (10(-12)-10(-4) mol/L) had no effect on either bone resorption or the number of multinucleate cells positive for tartrate-resistant acid phosphatase (TRACP + ve MNC) in OC cultured for 24 h on bone slices. However, in UMR/OC cocultures, 10(-4) mol/L zinc (but not lower concentrations) decreased resorption pit formation by approximately 50% and increased TRACP + ve MNC number by approximately 40%. When osteoblast-like cells were pretreated with zinc prior to, but not during, coculture with OC, effects on TRACP + ve MNC and pit number persisted, although the effect was reduced. Zinc treatment also inhibited resorption and stimulated TRACP and calcitonin receptor (CTR) + ve MNC numbers in long-term (96-120 h) UMR/OC cocultures. Our results indicate that zinc increases TRACP + ve CTR + ve MNC numbers yet inhibits bone-resorbing activity, and that these effects are dependent on the presence of osteoblastic cells. Zinc is abundant in bone and may act as a local regulator of bone cells.