1. Solutions of ethylhydrocupreine (optochin) hydrochloride show a pronounced bactericidal activity for pneumococci in pleural pus. 2. The highest tolerated dose of ethylhydrocupreine hydrochloride for guinea pigs by intrapleural injection is about 0.2 to 0.3 cc. of a 1:100 solution per 100 gm. of weight, corresponding to 2 to 3 cc. of solution or 0.02 to 0.03 gm. of the drug per kilo of weight. 3. The injection of 1 cc. of a 1:500 solution of ethylhydrocupreine hydrochloride into each pleural cavity of a guinea pig weighing from 350 to 500 gm. produces no evidences of toxicity or irritation of the pleura. 4. The injection of 1 cc. of a 24 hour dextrose blood broth culture of virulent Type I pneumococci into the right pleural cavity of guinea pigs produces acute suppurative pleuritis on both sides associated with suppurative pericarditis, which generally terminates fatally within 72 hours with pneumococcus bacteremia. 5. The injection of 1 cc. of 1:500 solutions of ethlylhydrocupreine hydrochloride into each pleural cavity of guinea pigs at varying intervals up to 24 hours after pleural infection has usually shown a marked curative influence. Similar results were observed with dogs. 6. The intrapleural injection of mixtures of ethylhydrocupreine, sodium oleate, and boric acid (Lamar) has also shown a decided curative effect in acute suppurative pneumococcus pleuritis of guinea pigs. 7. These and similar experiments on pneumococcus meningitis suggest that in chemotherapeutic investigations certain drugs may be injected into serous cavities in amounts exerting distinct bactericidal activity in vivo without producing local irritation or general toxic effects.