The specificity of a ribosomal system from human normal and malignant cervix uteri and a non analogous tRNA (calf liver) charged with (14C) phenylalanine by the synthetase from rat liver has been studied in relation to protein biosynthesis. The system is capable of incorporating phenylalanine into proteins, when directed by poly U in the presence of other factors including GTP. This indicates the specificity of interaction between macromolecular components, though they are non analogous. The resulting incorporation is greater under malignancy and highly sensitive to emetine but immune to mitomycin C and 'Jawaharene', a fungal antibiotic.