The intracellular mechanisms responsible for inhibition of osteoclast activity are of significant interest in the search for more effective ways of managing bone diseases associated with enhanced bone resorption. Previous studies have suggested that the protein kinase C (PKC) pathway is an important inhibitory second messenger in osteoclasts. We, therefore, investigated the effects of the synthetic peptide fragments, PKC(530-558) and (19-36), which correspond to parts of the catalytic and regulatory domains of PKC, on the activity of isolated osteoclasts. These fragments have been shown to activate and inhibit PKC, respectively, in biochemical studies employing isolated rat brain PKC, but have rarely been employed in studies of cellular activity. PKC(19-36), an enzyme inhibitor (PKC-I), had no effect by itself on osteoclastic bone resorption. However, PKC(530-558), a PKC activator (PKC-A), caused a dose-responsive inhibition of bone resorption, which was accompanied by a rapid and distinctive change in osteoclast morphology. This effect was reversible: (a) upon removal of PKC-A, (b) upon continuous exposure to this fragment for more than 36 h, or (c) in the presence of PKC-I. In conclusion, a short synthetic peptide fragment of PKC (PKC-A) significantly inhibits osteoclastic bone resorption; this, together with the fact that the inhibitory effect is abolished in the presence of PKC-I, provides further evidence for an important physiological role for the PKC pathway in the regulation of osteoclast activity. Selective activation of this pathway may have important therapeutic implications for the management of bone diseases associated with enhanced resorption.