At the request of the Department of Public Health of the city of Pittsburgh, the writer undertook to prepare Parke’s vaccine in large quantities. The vaccine was to be prepared under the direction of a committee consisting of Drs. Oskar Klotz, W. L. Holman, E. W. Willetts, George L. Hoffman and the writer, and the vaccine was to be turned over to the City Health authorities for distribution in the community. The work was carried out at the Singer Memorial Laboratory, and was begun the same day that the committee was appointed. Thirteen strains of Pfeiffer bacilli were used. Holman contributed six strains, isolated at autopsies done by Klotz at the Magee Hospital. Other fresh cultures were furnished by Willetts; Wiese, of the City Laboratory, and by the Singer Laboratory. The media used was that recommended by the New York Board of Health, save that sheep’s blood was used instead of horse blood because of convenience. The same technique was employed, with the exception that a modification of the Hopkins method of standardization was used instead of the Wright method. This was done because Pfeiffer bacilli are extremely small, tend to form unbreakable clumps and tangles, and so increase the difficulties of making satisfactory counts, either by means of the Wright method or with the Helber-Glynn counting chamber, that the methods are independable. Opalescent standards permit of such enormous variations that it was decided to use the Hopkins method, or a slight modification which we found so satisfactory that we will give our method here in detail. _Method of Standardization_