The bactericidal properties of "free" Cl, to the exclusion of all Cl-addition products, were studied in 56 series of tests with strains of Escherichia coli, Aero-bacter aerogenes, Pseudomonas pyocyanea, Eberthella ty-phosa, and Shigella dysenteriae exposed in waters of pH 7, 8.5, 9.8, and 10.7, and at 2 temp. ranges, 2[degree]-5[degree] and 20[degree]-25[degree] C. Practical application of the results may be made readily by reference to the tables and charts. Buffered Cl-demand-free waters were used in the tests. The results indicate in part that: 1) The time of exposure of bacteria to Cl is a primary factor governing the extent of the bacterial kill produced. This factor becomes particularly important when other conditions prevail which tend to impair or inhibit the bactericidal properties of Cl. 2) pH has a marked effect on the bactericidal efficiency of "free" Cl. At a temp. range of 20[degree]-25[degree] a Cl conc. of 0.1-0.29 ppm. in waters at pH zones of 7, 8.5, 9.8 and 10.7, required exposure times of 1, 10, 20, and 60 min. to produce a 100% kill of E. coli and 1, 3, 5, and 20 min. respectively for E. typhosa, while to produce a 100% kill of E. coli in 10 min. at a temp. range of 2[degree]-5[degree] and at pH 7, 8.5, and 9.8 required Cl concs. of 0.026-0.035, 0.1-0.29, and 0.7-0.99 ppm., respectively, and at pH 10.7 a Cl conc. of 1 ppm. produced a 100% kill only after approx. 20-60 min. of exposure. To obtain a 100% kill of E. typhosa at 2[degree]-5[degree] in 10 min. required about 0.03 ppm. of Cl at pH 7 and at pH 9.8 between 0.4 and 0.74 ppm. to produce similar results. The results suggest that the strains of E. typhosa used are more resistant to Cl than E. coli in the pH range of about 6.5-8 when the concs. of Cl used are 0.03 ppm. or less. At pH 8.5, 9.8 and 10.7 E. typhosa was more sensitive to Cl than E. coli. The possible reasons for this reversal in sensitivity are discussed. 3) Temp. also has an effect on the bactericidal properties of "free" Cl which is particularly marked when other factors, such as a high pH, are present that tend to reduce the bactericidal properties of Cl. E.g., at pH 7 with 0.03 ppm. of Cl present, both the rate and extent of kill of E. coli was somewhat greater at the temp. range of 20[degree]-25[degree] C than at 2[degree]-5[degree] C although with only 0.02 ppm. of Cl the extent of the kill at 2[degree]-5[degree] C was reduced markedly. However, at the higher pH ranges studied a decided effect was encountered. That is, a 100% kill of E. coli was obtained, (1) at pH 8.5 in 10 min. with about 0.07 ppm. at 20[degree]-25[degree] C, while about 0.14 ppm. were required at 2[degree]-5[degree] C, (2) in 5 min. at pH 9.8 with a Cl residual of about 0.3 ppm. at 20[degree]-25[degree] C while 0.72 ppm. required 10 min. at 2[degree]-5[degree] C, and (3) in 10 min. at pH 10.7 with about 0.4 pm. at 20[degree]-25[degree], and 1 ppm. at 2[degree]-5[degree] C did not produce the same result even in 20 min. The effect of temp. on the toxicity of Cl for E. typhosa was essentially the same as for E. coli. 4) At the pH zones of 8, 9.8 and 10.7 the strains of E. typhosa tested were more sensitive to Cl than E. coli or P. pyocyanea. In these pH ranges P. pyocyanea was the most resistant. At pH zones of 6.5, 7, and 7.8 with Cl concs. of 0.03 ppm. or less E. typhosa appeared to be slightly more resistant to Cl than E. coli or P. pyocyanea.