The rat osteogenic sarcoma sublcone UMR-106-01 is a cell type with osteoblst-like properties. This cell line has been shown to process specific receptors for insulin and insulin-like growth factor I (IGF-I), but not IGF-II. Insulin at physiological concentrations (1-5 ng/ml) in serum-free medium can maintain cell growth, as assessed by protein accumulation, thymidine uptake, and an increase in cell number. IGF-I is less potent than insulin, but, based on relative binding affinities for the insulin receptor, possible acts via its own receptor. Insulin also enhances PTH-stimulated cAMP accumulation in these cells both by increasing cell number and an effect independent of cell number. Insulin may have a role in bone homeostasis.