1. Ethylhydrocuprein hydrochloride in very high dilution inhibits the growth of, and in 18 hours kills, representatives of all four groups of pneumococci in vitro. The killing effect is generally seen in somewhat lower dilutions than the inhibiting effect. No constant or considerable difference is seen in these actions on representatives of the four groups of the pneumococci. The action of ethylhydrocuprein hydrochloride on the pneumococci in vitro is so strongly specific that it may possibly be used as a test for a true pneumococcus. 2. The inhibitory or killing effects of ethylhydrocuprein hydrochloride in vitro on bacteria other than pneumococci are slight or absent. The effects are greater on streptococci than on any other organisms examined, but are still much less than on the pneumococci. This action distinguishes between the streptococcus group, including Streptococcus mucosus sometimes found in normal mouths, on the one hand, and the true pneumococcus (including Pneumococcus mucosus), on the other. 3. Quinine hydrochloride inhibits the growth of, and kills the pneumococcus in vitro; much stronger concentrations, however, are necessary than in the case of ethylhydrocuprein. This effect of quinine hydrochloride is also seen on other organisms, but in a less degree. 4. Ethylhydrocuprein (optochin base) has a well marked protective action against experimental pneumococcal infection in mice in the case of type strains of all four groups of pneumococci; this protective action may be efficient against many multiples of the minimum lethal dose.