The color reaction for detection of lactic acid was investigated for application to a preliminary test for demonstration of sweat stains. The detection of lactic acid was employed with the commercially available “Determiner-LA” kit (KYOWA medics).   The positive reaction was indicated by the development of a red-purple color. The positive results could be observed in sweat stains and also seminal stains. The weak positive reaction could be seen in only one of 20 salivary stains but all urinary stains gave negative results.   Therefore, it is possible to discriminate sweat stains from urinary and salivary stains by using this preliminary test. However, for the distinction between sweat and seminal stains, the acid phosphatase test (SM-test reagent) should be examined. This preliminary test was detectable from sweat stains stored for 6 months at room temperature and from stains of 16-fold diluted human sweat. This preliminary test is very useful for the demonstration of sweat stains in forensic practice.