GR2H6 is a fibroblast-like cell line cloned from a GR mouse strain mammary tumor, which can grow in a chemically defined serum-free medium. In this serum-free medium, we have found that GR2H6 cells require insulin or insulin-like growth factor I (IGF-I) and epidermal growth factor (EGF) and respond Oo these factors in a dose-related manner as measured by the increase of cell number and total DNA. Based on these observations, me have see up a bioassay system for the growth promoting activities of insulin, EGF and IGF-I. Compared with other bioassay systems this system has several advantages, such as higher sensitivity, better reproducibility and feasibility for different factors. It may provide a novel model system to study growth promoting activities of insulin and other growth factors.