The amount of chlorine required for efficient treatment is very largely determined by the amount required to satisfy the oxidisable matter present in the water. Many experimenters have reported results that would indicate that appreciable concentrations of chlorine are required for bactericidal action but the details of the technique, as published, show that the effect of the organic matter added with the test organism was not thoroughly appreciated. One cubic centimetre of a culture in ordinary peptone water, added to one litre of water, would increase the organic content by approximately 10 parts per million, an amount that would absorb appreciable amounts of chlorine. Other conditions also make it very difficult to compare the results obtained in the past: one of these is the degree of purity set as the objective. German bacteriologists added enormous numbers of the test organism and endeavoured to obtain the complete removal of the organism from such quantities as one litre of water with a contact period often as short as 10 minutes. Nissen,[1] of the Hygienic Institute of Berlin, found that a 1: 800 dilution of bleach (420 p.p.m. of chlorine) was required to destroy _B. typhosus_ in one minute and a 1:1600 dilution (210 p.p.m. of chlorine) in 10 minutes. Delépine[2] obtained somewhat similar results by means of the thread method for testing disinfectants. Phelps,[3] using gelatine plates for enumeration of the bacteria, obtained a 90 per cent reduction of _B. typhosus_ in twenty minutes with 5 p.p.m. of available chlorine; over 99 per cent reduction in one hour, and over 99.99 per cent reduction in 18 hours. Wesbrook, Whittaker, and Mohler[4] tested bleach solutions with various strains of _B. typhosus_ by means of the plate method and found that the most resistant one was reduced from 20,000 per c.cm. to sterility (in 1 c.cm.) by 3 p.p.m. of available chlorine in fifty minutes and that the least resistant one only required 1.0 p.p.m. with a thirty minutes' contact. Lederer and Bachmann[5] have reported the following results: TABLE V PERCENTAGE REDUCTION, 15 MINUTES' CONTACT