Insulin raises the rate of incorporation of [3H]Leu into the total protein of hormone-deficient chick embryo fibroblasts by .apprx. 1.5-fold. The elevation is not dependent upon the production of new mRNA. Evidence is now presented in support of the following points:the greater labeling is due to more rapid polypeptide synthesis, not to an increase in the specific activity of leucyl-tRNA; the enhanced synthesis derives largely or entirely from a speeding up of the process of initiation, rather than that of elongation or termination; and the 1.5-fold stimulation is due to the elevated rates of formation of at least many of the fibroblast proteins. The hormone was shown before to stimulate posttranscriptionally and highly preferentially the formation of ribosomal proteins in the resting chick embryo cells. Whether insulin increases the production of total cell ribosomal protein by chemically altering preformed mRNA was studied. Results obtained by translating messages from deprived and hormone-treated cells in wheat germ and reticulocyte preparations do not support a mechanism involving covalent modification of preformed mRNA. These observations and those previously made with inhibitors of translation suggest that insulin stimulates protein synthesis in the resting chick embryo cells by activating limiting components of the initiation system. The effects of the hormone are greatest with messages, such as those for the ribosomal proteins, that have low affinities for the limiting initiation components.