Studies have been carried out to evaluate the effect of mixtures of the amino acid analogues p-fluorophenylalanine (PFPA) and ethionine (E) on the life-span (proliferative capacity in terms of total population doublings achieved) of human diploid cell line W1-38. Cells grown continuously in the presence of low, “nontoxic“ concentrations of the analogues (1.75 × 10−6 /μm PFPA; 0.92 × 10−6 μM E) incorporated the analogues into protein but were able to achieve the same number of population doublings as untreated controls. When the cells were exposed to concentrations of the analogues which clearly inhibited proliferation (175 μM PFPA and 92 μM E) for 1 week and then placed in fresh analogue-free medium, the cultures recovered and were able to achieve essentially the same number of population doublings as the controls.These findings do not seem to support the error catastrophe as a basis for aging in proliferating cell cultures.