Phage T4 lysozyme consists of two domains between which is formed the active-site cleft of the enzyme. The crystallographically determined thermal displacement parameters for the protein suggested that the amino terminal of the two domains undergoes 'hinge-bending' motion about an axis passing through the waist of the molecule. Such conformational mobility may be important in allowing access of substrates to the active site of the enzyme. We report here a crystallographic study of a mutant T4 lysozyme which demonstrates further the conformational flexibility of the protein. A mutant form of the enzyme with a methionine residue (Met 6) replaced by isoleucine crystallizes with four independent molecules in the crystal lattice. These four molecules have distinctly different conformations. The mutant protein can also crystallize in standard form with a structure very similar to the wild-type protein. Thus the mutant protein can adopt five different crystal conformations. The isoleucine for methionine substitution at the intersection of the two domains of T4 lysozyme apparently enhances the hinge-bending motion presumed to occur in the wild-type protein, without significantly affecting the catalytic activity or thermal stability of the protein.