SUMMARY

The effect of amino acid concentration on the initial growth rate of the contact-inhibited mouse fibrobla8ts 3T3 and its SV40-transformed derivative 3T3T has been studied. In medium in which the serum had been passed through a Sephadex column, and the concentration of amino acids lowered 1000-fold, one acid at a time, the growth rate constants were computed. For most of the 16 amino acids studied, the initial growth rate of 3T3 decreased more pro-foundly than that of 3T3T. The presence in serum of a factor which stimulates amino acid uptake and growth rate of 3T3T is described