The specificity and mechanism of the Scott reaction for cocaine screening were studied. This reaction consists of three procedures : 1) addition of cobaltous thiocyanate/glycerol, 2) addition of hydrochloric acid, and then 3) mixing the solution with chloroform. Our experiment using 30 drugs showed that the precipitate-forming reaction of the first step above was not specific for cocaine. However, the color reaction using the three steps was highly specific for cocaine, at least in the test using low amounts (<1.0 mg) of drugs. Promazine, promethazine, chlorpromazine, imipramine, lidocaine and diltiazem showed positive reaction when 5 mg or more of these drugs were used. These findings indicated that the specificity of the Scott reaction is markedly affected by the amount of a drug used. The stoichiometric study indicated that cocaine binds to cobaltous thiocyanate at a molar ratio of 2 : 1. Based on this finding together with the specificity of the Scott reaction, we postulated a structure of cocaine-cobaltous thiocyanate.