PTH-related protein (PTHrP) interacts, via its amino-terminal 34 residues, with PTH receptors on osteoblasts to stimulate osteoclastic bone resorption indirectly. We now report that mature hPTHrP-(1-141) (EC50, approximately 10(-11) M) and a carboxyl-terminal fragment, PTHrP-(107-139) (EC50, approximately 10(-15) M), are potent inhibitors of resorption in an isolated rat osteoclast bone resorption assay, whereas hPTHrP-(1-108) and hPTHrP-(1-34) are inactive in this respect. PTHrP-(107-139) also inhibits resorption in a rat long bone organ culture system and reduces osteoclast spreading. PTHrP-(107-139) does not act on osteoclasts via a cAMP signal transduction mechanism, but its effects may be mediated by protein kinase-C. This previously unrecognized action of PTHrP, to inhibit osteoclastic bone resorption directly, indicates that PTHrP may be a precursor of multiple biologically active peptides with differing physiological functions.