While the above illustrates the status of “mixed vaccine” for therapeutic purposes, it is a well-recognized fact that it is possible to produce an immunity for most of the bacteria composing such vaccines, if killed cultures of the various strains are injected in sufficiently large doses. Again referring to Pearce’s article, we find the statement: “Prophylactic vaccination rests on a sound, scientific basis of experimental studies and clinical observation.” The attempt to protect against epidemic influenza by the use of mixed vaccines was based largely on the following points. The medical profession was confronted by a rapidly approaching deadly epidemic, against which ordinary measures of control had failed. The epidemic was supposed to be due to a primary infection with Pfeiffer’s bacillus, but all of the fatal cases were found to have profound secondary or symbiotic infections, with one or more of the strains contained in the “mixed vaccines.” It was known that mixed bacterial proteins, even though they were not actually specific, possessed certain qualities of producing reactions unfavorable to infections in general, which were characterized by a temporary rise in temperature, by an increase in the number of leucocytes, and by a more or less demonstrable amount of active immunity against each one of the contained bacterial toxins. The artificial production of a leucocytosis was especially desirable, because a characteristic of epidemic influenza was the failure of leucocytosis on the part of the infected individual. In other words, mixed vaccines were used because they were the only available substances which offered the hope of creating a reaction against the secondary invaders which were so commonly the cause of death in influenza. Since Pittsburgh’s experience with prophylactic vaccination had chiefly to do with the use of commercially prepared mixed vaccines, a brief history of the local experience with them may be of interest.