For that purpose it seemed rational to attempt the treatment with a vaccine containing a combination of bodies of microbes, together with their toxic products. I intended to test this plan experimentally in the cholera districts; but, plague having broken out in Bombay, the Government of India commissioned me to inquire into the bacteriology of that disease, and I determined that the knowledge gained in the cholera inoculations should be applied and tested in the preparation of a prophylactic against the new epidemic. The experiments I had in view involved manufacturing a material on a large scale, and operating on it for weeks continuously. To do this it was essential to find a way of recognizing plague growth with certainty, so as to enable the officers engaged in the manufacture to control the process and know exactly when they were handling the proper stuff, and when an admixture and invasion of extraneous growth took place. When this was solved, a drug was prepared by cultivating the plague microbe in sterilized broth, to which a small quantity of clarified butter or of cocoanut oil had been added. The plague bacilli attach themselves to the drops of butter or oil floating on the surface, and grow down into the depth of the liquid, forming a peculiar threadlike appearance. While doing so they secrete toxic matter, which is gradually accumulated in the liquid; at the same time a large amount of microbial growth comes gradually down from the surface of the liquid and collects at the bottom of the flask. When shaken up the whole represents the desired combination of the bodies of microbes and of their toxic products. The process is continued for a period of five to six weeks. As the microbes of plague had been very little studied before, and as their exact effect on the human system was unknown, I decided not to use for the treatment living microbes, but to use at least at first ‘carbolized’ vaccines, though the result of the treatment might be less favorable or less lasting than that which could be expected from living vaccines. The microbes in the above plague growth were accordingly killed by heating them at a temperature ranging from 65° to 70° C., and then mixed with a small proportion of carbolic acid, to prevent the drug from subsequent contamination and decomposition. The dose of the prophylactic was regulated by measuring up the quantity to be injected. The requisite amount is determined by the degree of fever which it produces. The febrile reaction varies in different individuals, but a temperature reaching 102° and above in at least thirty per cent of those inoculated has been found to indicate a good material. In the cholera, rabies and smallpox vaccines, the microbes being employed in a living state, it was essential to fix the strength of the vaccine, for otherwise it was impossible to predict the behavior of the microbe when injected into the system. In the case of