Nearly all the zinc in oysters is bound, either to soluble high-molecular weight proteins or to structural cellular components such as cell membranes. Oyster alkaline phosphatase is a zinc metalloenzyme, as indicated by in vitro inhibition studies with various metal-binding agents. Dialysis of soluble tissue extracts atpH 7–9 removes up to 96% of the total zinc without effect on alkaline phosphatase. If alkaline phosphatase is considered representative of the metabolic functions of zinc in oysters, most of the zinc accumulated by oysters must be superfluous to the animal's requirements.