Thialysine, the lysine isolog with the 4-methylene group substituted by a sulfur atom, inhibits the growth rate and plating efficiency of Chinese Hamster Ovary (CHO) cells. The inhibition can be reversed by lysine, when added to the culture medium together with thialysine or shortly after; to have a complete reversion a lysine concentration five times that of thialysine is necessary. Cells grown in the presence of thialysine show a decreased viability and an increased volume. Thialysine inhibits protein synthesis, while it does not affect DNA and RNA synthesis. Protein synthesis inhibition can be reversed by lysine. Overall the results obtained indicate that thialysine affects cellular functions by impairing lysine utilization.