1. Russian encephalitis and louping ill viruses showed a close relationship by complement fixation, neutralization, and intraperitoneal cross-resistance tests. Intracerebral cross-resistance tests, on the other hand, failed to reveal any connection between them. Neither Russian nor louping ill virus appeared to be related to the remaining viruses tested.

2. Japanese B, St. Louis, and West Nile types of encephalitis, as a group, showed a certain degree of group relationship, but it was not so close as that between the Russian and louping ill viruses. In complement fixation tests, besides homologous reactions, Japanese serum brought about reactions with both St. Louis and West Nile antigens; St. Louis serum reacted with Japanese antigen, and West Nile serum with Japanese antigen. In neutralization tests with mouse sera, no relationship was found amongst these three viruses, while in similar tests with either hamster or guinea pig serum,—which gave higher homologous titers,—it was found that Japanese serum protected against West Nile and St. Louis viruses, St. Louis serum protected against West Nile virus, and West Nile serum against Japanese virus. In intracerebral and intraperitoneal cross-resistance tests, no relationship was found to exist between these three viruses. Moreover, the Japanese B, St. Louis, and West Nile viruses appeared to be unrelated to any one of the three other viruses tested.

3. W.E.E. virus stood apart in all tests as unrelated to any of the other viruses studied.

4. The homologous titers of complement-fixing antibodies in mouse sera showed a gradual decline with the passing of time after vaccination, and this loss of homologous titer was paralleled by a similar drop in the titer of the heterologous reactions. In the case of the Japanese B, St. Louis, and West Nile viruses, with which at the start the amount of crossing was not high, a point was reached beyond which heterologous reactions could no longer be detected.

5. A comparison of the specific levels of complement-fixing and neutralizing antibodies for the viruses in mouse hyperimmune sera showed their rate of persistence to differ. Complement-fixing antibodies which had highest titers on the 10th day, diminished gradually until, when tested on the 50th day, all titers had reached levels from one-fourth to one-eighth of their values on the 10th day. On the other hand, the levels of neutralizing antibodies for the same samples of serum were, on the 50th day, as high as or higher than those found on either the 25th or 10th days, save in the case of the Japanese B virus.

6. The state of immunity of animals following vaccination with the viruses discussed was found to be different at a given time, depending on the method employed to determine it. Thus, mice vaccinated with St. Louis virus had positive complement-fixing antibodies on the 10th day and no neutralizing antibodies. The state of immunity changed in the course of time. For this reason it is felt that in order to detect whether two viruses are related or not, multiple observations are necessary, over a considerable time and employing all available methods of immune comparison.