We have identified a novel factor which stimulates the formation of osteoclasts and which is produced by a human tumor as well as by leukocytes which have been stimulated with concanavalin A. We have called this peptide osteoclastpoietic factor (OPF). We have obtained amino acid sequence on this peptide and found that it is novel and distinct from other known growth regulatory factors, interleukins, and colony stimulating factors. We have found that a synthetic peptide which corresponds to this amino acid sequence mimics the biological effects, and we have produced both monoclonal and polyclonal antibodies which neutralize the original biological activity. In this project, we plan to: 1. Clone the gene and express the recombinant molecule. 2. Determine the role of OPF in normal bone remodeling and in disease states associated with increased bone resorption. Characterization of novel cytokines which are powerful stimulators of osteoclastic bone resorption is likely to lead to new insights in the normal control of osteoclast function and in the pathophysiology of disease states associated with bone destruction.