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Serological techniques have been used to detect small changes in amino acid sequences of specific proteins in closely related biological systems. These changes are believed to reflect evolutionary divergence. In the present study, the differences in binding capacities of catalases from different mycobacteria for a reference antiserum were measured very specifically by assaying the unbound functional enzyme after exposure to antibody. Catalase derived from Mycobacterium tuberculosis has been used to immunize rabbits. The antibody so produced precipitated the enzyme but did not inactivate it. This antibody also precipitated catalase from sonic lysates of other mycobacterial species. The binding capacity of catalase derived from a number of heterologous species for the M. tuberculosis antibody was always lower than that of homologous enzyme. Some species produced catalase that failed to react at all with the reference serum. In other cases, there was evidence of at least two serologically unrelated catalases in a single strain. There was also a limited correlation between rate of inactivation of catalase by heat and the relative antibody-binding capacity of the enzyme. The serological study of catalases offers promise of providing a useful tool for clarifying evolutionary relationships among mycobacterial species.