a way that it is not killed but merely weakened. _Black-leg_ vaccines are made by this method. (5) Chemicals are sometimes added to attenuate the organisms, as was formerly done in the preparation of black-leg vaccine by Kruse’s method in Germany. The use of toxin-antitoxin mixtures in immunizing against diphtheria and in the preparation of diphtheria antitoxin from horses is an application of the same principle, though here it is the _product_ of the organism and not the organism whose action is weakened. (6) Within the past few years the workers in the Pasteur Institute in Paris have been experimenting with vaccines prepared by treating living virulent bacteria with antisera (“sensitizing them”) so that they are no longer capable of causing the disease when introduced, but do cause the production of an active immunity. The method has been used with typhoid fever bacilli in man and seems to be successful. It remains to be tried out further before its worth is demonstrated (the procedure is more complicated and the chance for infection apparently much greater than by the use of killed cultures). The term _sero-bacterins_ is used by manufacturers in this country to designate such bacterial vaccines. (7) Growing on artificial culture media reduces the virulence of most organisms after a longer or shorter time. This method has been tried with many organisms in the laboratory, but is not now used in practice. The difficulties are that the attenuation is very uncertain and that the organisms tend to regain their virulence when introduced into the body. In producing active immunity against many bacterial diseases the organisms are introduced (_c_) dead. They are killed by heat or by chemicals, or by using both methods (Chapter XXX). When the products of an organism are introduced the resulting immunity is against the products only and not against the organism. If the organism itself is introduced there results an immunity against it and in some cases also against the products, though the latter does not necessarily follow. Hence the immunity may be _antibacterial_ or _antitoxic_ or both.