Baby hamster kidney cells (BHK) were able to proliferate in the complete absence of serum in synthetic medium supplemented with insulin, transferrin, fibroblast growth factor and epidermal growth factor on cell culture dishes coated with fibronectin. Although the addition of individual supplements had little or no effect on cell growth, the combination of the supplements resulted in a significant synergistic effect. The defined medium was also capable of supporting the clonal growth of BHK cells in the absence of serum. Studies on the effect of hormonal supplementation in the presence of low concentrations of fetal calf serum suggest that the function of serum is to supply hormones and growth factors for cell growth.