Here we show that the substitution Thr 26-->His in the active site of T4 lysozyme causes the product to change from the alpha- to the beta-anomer. This implies an alteration in the catalytic mechanism of the enzyme. From the change in product, together with inspection of relevant crystal structures, it is inferred that wild-type T4 lysozyme is an anomer-inverting enzyme with a single displacement mechanism in which water attacks from the alpha-side of the substrate. In contrast, the mutant T26H is an anomer-retaining enzyme with an apparently double displacement mechanism in which a water molecule attacks from the opposite side of the substrate. The results also show that the mechanism of wild-type T4 lysozyme differs from that of hen egg-white lysozyme even though both enzymes are presumed to have evolved from a common precursor.