The Demonstration of the Increase of Globulin in Diphtheria Antitoxins by the Precipitation Reaction.

1. By titration against horse serum and its protein fractions anti-horse sera can be divided into three groups, according to the antibodies responsible for the main or earliest particulating zone, namely the sera containing antibodies against ( a ) globulin only, ( b ) albumin only, and ( c ) both globulin and albumin. 2. For the quantitative estimation of a protein fraction in a horse serum it is necessary to use an antiserum containing a main zone antibody acting with that protein only. This is illustrated in experiments, designed to show the increased globulin content of diphtheria antitoxin, in which antisera containing antibodies against both globulin and albumin failed to register an increase in the globulin content.