Ever since artificial media have been used for the cultivation of micro-organisms, the question as to what is the best reaction—that is the degree of acidity or alkalinity—has been a moot point. It has been assumed gratuitously that there is an optimum reaction which is the same for all the common bacteria met with in the laboratory. In recent years however some attempts have been made to settle the question definitely, notably by Eyre and Deelemann. On enquiring what reaction of medium was adopted in six different London laboratories, Eyre found that very divergent standards were used, ranging from 20 units of acidity to 10 units of alkalinity on his scale. (20 units of acidity implies that 20 c.c. of normal alkali must be added to each litre to make it neutral to phenolphthalein. In the case of alkalinity the indicated number of c.c. of normal acid must be added.) The importance of having a definite reaction had until recently not impressed itself upon the majority of bacteriologists. The possibility that although many organisms may grow on media of comparatively widely differing reactions, yet only one of these reactions may be suitable for the growth of a more delicate kind, had been overlooked. Consequently they failed to grow bacteria which by careful adjusting of the reaction of the medium could be readily cultivated. Martin (1910), for example, has lately pointed out the ease with which gonococcus can be made to grow on serum agar if the reaction is carefully adjusted. In later years the subject has assumed still more importance in connection with the production of bacterial toxins, which have been submitted to a large amount of investigation. In the production of diphtheria toxin it was soon recognised that bouillon containing more alkali than is usually employed for ordinary bacteriological work, yielded the most toxin, and even various devices were invented to avoid the initial production of acid which usually takes place as the consequence of bacterial growth in a sugar-containing medium. Madsen (1896) has made a thorough examination of the factors governing the production of diphtheria toxin. By following the reaction of a growing culture from day to day he found two different types of growth occur,—one in which the medium increases in acidity without abatement, and such cultures are atoxic; in the other the primary increase in acidity is followed by a decrease, and subsequent increase in alkalinity. These cultures are sometimes toxic. The two conditions seemed to have relation to the amount of alkali added in the preparation of the bouillon, in that a strongly alkaline medium gave an alkaline end reaction while a less alkaline medium gave sometimes an acid and sometimes an alkaline end reaction.