1. An electrophoretic study has been made of normal mouse serum, influenza mouse serum, and influenza mouse lung suspension. The mobilities of the protein fractions present have been determined at various pH's by the optical method.
2. The pH-mobility curve of the soluble (complement-fixing) antigen present in the lung suspension has been determined analytically by sampling and application of the complement fixation test. The results show that the complement-fixing antigen has a mobility definitely smaller than that of serum albumin and close to that of a globulin, with an isoelectric point close to pH 5.