The growth-promoting activities of fetal bovine serum, cortisol, phorbol myristate acetate, prostaglandin F2alpha, insulin, epidermal growth factor, and fibroblast growth factor were evaluated on four murine embryo cell lines (Swiss 3T3, Balb 3T3, M2, and C3H10T 1/2). Each cell has an unique response spectrum to this collection of reported mitogens. Phorbol myristate acetate and prostaglandin F2alpha were active only on selected cell lines; cortisol was inactive on all four lines. Serum, epidermal growth factor, and fibroblast growth factor were able to stimulate cell division in all four lines, albeit to varying degrees for the different target cells.