A plasmid-borne gene encoding bacteriophage T4 lysozyme with a structural mutation, Tyr161-Ala, was mutagenized by by the use of polymerase chain reaction. The mutagenized gene was inserted into a specialized bacteriophage lambda cloning vector that must acquire a functional lysozyme gene in order to form plaques. Functional variants of the mutant lysozyme were selected. Three compensatory second-site revertants were obtained: Thr152-Met, Lys43-Ile, and Thr151-Ala. The effects of these mutations are interpreted in light of previous structural and genetic studies of T4 lysozyme.