Evidence has been submitted of the existence of a common antigenic substance in horse dander and horse serum. This evidence has been derived from three lines of study:
(a) Cross-precipitation tests involving (1) the titration of antisera against horse serum with saline extract of horse dander and (2) titration of antisera against horse dander with normal horse serum.
(b) Cross-anaphylactic tests by the uterine strip method of Dale.
(c) Cross-anaphylactic tests in living guinea pigs by the usual shock method.
It seems likely from the work here described that the common antigen is present in small proportion in horse dander. Its concentration in horse serum is not indicated by the results obtained.