Osteoclastic acid phosphatase is a member of a widely-distributed class of iron-containing proteins with acid phosphatase activity. Antibodies raised against one member of this class cross-react with other members from the same or different species, but not with acid phosphatase isoenzymes of different types. When antibodies to one such protein, porcine uteroferrin, are added to medium in which rat osteoclasts are incubated on devitalised cortical bone, both bone resorption and acid phosphatase activity are markedly inhibited. Furthermore, addition of molybdate (an inhibitor of this class of acid phosphatases) also inhibits both bone resorption and enzyme activity. These observations strongly suggest a functional role for osteoclastic acid phosphatase in bone resorption.