Most workers outside the Wright school have failed to recognize any essential value of determinations of the opsonic index in the use of vaccines. Some of the reasons for this are as follows: The limit of error in phagocytic counts may be as great as 50 per cent. in different series of fifty, hence several hundred must be counted, which adds greatly to the tediousness and time involved; the variation in apparently healthy individuals is frequently great, hence the “normal” is too uncertain; finally the opsonic index and the clinical course of the disease do not by any means run parallel. Undoubtedly the method has decided value in the hands of an individual who makes opsonic determinations his chief work, as Wright’s assistants do, but it can scarcely be maintained at the present time that such determinations are necessary in vaccine therapy. Nevertheless that opsonins actually exist and that they play an essential part in phagocytosis, and hence in immunity, is now generally recognized. BACTERIAL VACCINES. Whether determinations of opsonic index are useful or not is largely a matter of individual opinion, but there is scarcely room to doubt that Wright has conferred a lasting benefit by his revival of the use of _dead cultures of bacteria_, _bacterial vaccines_, both for protective inoculation and for treatment. It is perhaps better to use the older terms “vaccination” and “vaccine” (though the cow, _vacca_, is not concerned) than to use Wright’s term “opsonic method” in this connection, bearing in mind that the idea of a vaccine is that it contains the _causative organism_ of the infection as indicated on p. 253. As early as 1880 Touissant proposed the use of dead cultures of bacteria to produce immunity. But because injections of such cultures were so frequently followed by abscess formation, doubtless due to the _high temperatures_ used to kill the bacteria, the method was abandoned. Further, Pasteur and the French school persistently denied the possibility of success with such a procedure, and some of them even maintain this attitude at the present time. The successes of Wright and the English school which are being repeated so generally wherever properly attempted, leave no doubt in the unprejudiced of the very great value of the method and have unquestionably opened a most promising field both for preventive inoculation and for treatment in many infectious diseases. That the practice is no more universally applicable than are immune serums and that it has been and is still being grossly overexploited is undoubted.