By means of a method differing in important details from those of previous investigators it has been determined that the blood serum of cases of leprosy exhibits the ability to fix complement with a wide variety of antigens including to a greater or less extent those derived from any culture of the acid-fast group of bacteria available to us.

This property of multiple fixation may sufficiently characterize the disease to be of diagnostic significance, although our experience is hardly sufficient to enable us to speak with complete assurance on this point. Certainly, control sera from normal individuals, from cases of tuberculosis, or from cases of syphilis as obtained in our locality have entirely failed to react with certain antigens, whereas serum from cases of leprosy have so reacted to the extent of over 93 per cent.

The most characteristic fixation given by the leprosy sera is that with Bacillus lepræ (Clegg) used as antigen, either in the form of a bacterial emulsion or of an alcoholic extract of the dried culture.

Antibody absorption may be demonstrated in the acid-fast group if the absorbing bacteria are removed by filtration. Otherwise the resulting fluid is strongly anticomplementary. Leper serum is not deprived of the complement-fixing body when so treated with either Bacillus tuberculosis or Bacillus lepræ (Clegg).

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