Abstract Sodium lauryl sulfate rapidly cleared suspensions of pneumococci with disintegration of the organisms. Several hundred strains of pneumococci of various types have been studied and found to be lysed rapidly in a 0.2 per cent concentration of the compound. All freshly isolated strains showed definite clearing within thirty minutes. Streptococci under the same conditions were unaffected. The test is best performed by adding 0.1 c.c. of a 2 per cent solution of crude sodium lauryl sulfate (“Dreft”) to 0.9 c.c. of a 24-hour culture. Usually the results can be determined in a few minutes, but if clearing does not occur, it is necessary to incubate the mixture at 37 °C. for one-half hour before the final reading.