Metal complexes of trimethylamine carboxyborane successfully suppressed calcium flux from both paired pup calvaria bones and rat UMR-106 osteosarcoma cultured cells over a 48 h period. These agents increased uptake of calcium into the cell cultures and accelerated ['Hlproline incorporation into collagen. Copper and iron complexes of the trimethylamine carboxyborane were more potent compared with the cobalt and chromium complexes. The agents effectively reduced lysosomal enzyme activity and also proteolytic enzyme activities of macrophages. Since macrophages invade the bone surface and assist in the demineralization and digestion of collagen, those agents may be potentially useful to retard diseases involving bone reconstruction. Influx of white blood cells and macrophages to sites of degradation most probably would be inhibited by the agents, based on sponge test observations in mice. Osteoporosis induced by ovariectomy was minimized by injections of tetrakisrp-(trimethylarnine-boranecarboxylato)bis( trimethylamine-carboxyborane)dicopper(I1) into rats at 3.5 mg kg-' day-' for 14 days. Bone volume, density, weight and calcium content returned to normal baseline control values. In addition, the copper complex returned serum calcium, serum parathyroid hormone (PTH) and vitamin D, values to normal levels. One possible mode of action of these derivatives is the regulation of the production and release of chemical mediators initiating bone loss, e.g. tumor necrosis factor, TNFa and interleukins I1 or 11-2.