The use of animal serum in cell culture is vital for providing the nutrient factors required to promote proliferation and function. Fetal calf serum has become the preferred choice because of its abundance, reasonable cost, and ability to sustain human cells in vitro. Although a wide variety of serum sources have been tested and used, little is known about the ability of serum obtained from the American black bear (Ursus americanus) to support human cell growth in culture. The American black bear, an animal comparable in size to humans, is unique in that it hibernates for mo at a time but does not experience extensive bone loss normally associated with extended immobility. The aim of this study was to analyze the effect of bear serum on human osteoblast cultures. We discovered that three of the eight bear serum samples induced significantly higher proliferation rates in osteoblasts than did fetal calf serum over a 24-h period. Osteoblasts incubated in bear serum displayed higher messenger ribonucleic acid levels for phenotype markers osteocalcin and type I collagen than did those incubated in fetal calf serum. The mitogenic activity of the bear serum was reduced when heated at 56° C for 30 min before use in culture. The molecular weight of the mitogenic factors was found to be primarily greater than 50 kDa. The present work demonstrates the capability of serum from American black bears to support human osteoblast proliferation in vitro.