The fidelity of protein synthesis was measured in human diploid skin fibroblasts as a function of passage level ("aging in vitro") and physiological age of tissue donor ("aging in vivo") using two different test systems. First, in cell-free extracts the ratio of delta leu/delta phe incorporation into peptide linkage following in the latter case using cells derived from elderly normal donors and from subjects with the premature aging disorders of Hutchinson-Gilford progeria and the Werner syndrome. Similar results were obtained using a second system of intact cells whereby histidine starvation induces quantifiable satellite spots resolved by two dimensional electrophoresis on polyacrylamide gels on the acidic side of the native actin species due to substitution of the neutral amino acid glutamine for the basic histidine. In fact, error frequencies appeared to decrease during aging in vitro, likely due to selection for clonal subpopulations with the highest fidelity of protein synthesis. The only increases were seen in the intact cell system where SV40-transformed cells showed a three-to-five fold greater error frequency compared to nontransformed fibroblasts. In total, these data fail to support the error catastrophe theory of cellular aging.