A new method has been developed that makes it possible to site-specifically incorporate unnatural amino acids into proteins. Synthetic amino acids were incorporated into the enzyme β -lactamase by the use of a chemically acylated suppressor transfer RNA that inserted the amino acid in response to a stop codon substituted for the codon encoding residue of interest. Peptide mapping localized the inserted amino acid to a single peptide, and enough enzyme could be generated for purification to homogeneity. The catalytic properties of several mutants at the conserved Phe66 were characterized. The ability to selectively replace amino acids in a protein with a wide variety of structural and electronic variants should provide a more detailed understanding of protein structure and function.