A simple and sensitive method for the quantitative determination of organic mercury in waste water, fish tissues, and sea or river bottom sediments is described. This method consists of extraction of organic mercury from the material with benzene, reverse-extraction of the organic mercury with cysteine acetate solution, and combustion of the solution in the system described earlier. This method has the following features : (1) It is not necessary to use organic mercury such as methylmercuric chloride, ethylmercuric chloride, butylmercuric chloride, or phenylmercuric acetate, as the standard substance in the determination of organic mercury compounds, and inorganic mercuric chloride can be used as the standard in this method. (2) Organic mercury compounds can be completely separated from inorganic compounds by using benzene and cysteine acetate solution. (3) Analytical values of organic mercury compounds obtained by the proposed method agree completely with that obtained by the gas chromatographic analysis with electron captuer detector (ECD-GC method). (4) In carrying out this proposed method, it is not necessary to obtain any licence for handling radioactive rays which is required in the case of the ECD-GC method. Satisfactory results have been obtained by applying this proposed method to the analysis of organic mercury in various materials.