We have developed a novel method to examine the translational fidelity of mammalian ribosomes in vitro, where protamine mRNA was used as a template. This method enabled us to determine frequency of misrecognition of purine bases at the second position of arginine codons (AGR/AAR) in the mRNA. Using this method the fidelity of translation of ribosomes derived from mouse livers was found to remain unchanged from 2 to 29 months, the maximum life span of the animal. This conclusion is not consistent with the "error catastrophe" theory of aging. This is the first report in which translational fidelity of ribosomes of animals of various ages has been compared by an in vitro translation of a natural mRNA.