The origin and nature of gas gangrene can be diagnosed exactly only by time-consuming bacteriological tests. In order to improve the diagnostic procedures, rabbits were infected with strains of Clostridium perfringens, Clostridium septicum or Clostridium sordellii. Sialidase activity was found to increase rapidly in serum; elevated creatine kinase activities were observed, too. High sialidase concentrations were found in sera (up to 1.6 mU/ml) and in tissues of wounded regions (up to 110 mU/g) of patients diagnosed to be infected with C perfringens. By inhibition of enzyme activity with antibodies specific for the sialidase from this Clostridium species, it was possible to identify the clostridial origin of the sialidase activities. In the same material from other patients supposed to suffer from gas gangrene, but where no Clostridia could be detected, significant sialidase activity was not found. Thus, sialidase may be a useful tool for the diagnosis of myonecrosis due to clostridial infection.