A sensitive method for the identification and quantitation of the organophosphate insecticide oxydemeton-methyl and its metabolite demeton-S-methylsulfon from human blood and various tissue samples has been established. After solid-phase extraction using C18 cartridges the extracts were analyzed by high-performance liquid chromatography with mass selective detection (Finnigan MAT LCQ). The method was completely validated. The limit of detection is 1 ng/g blood for the parent substance oxydemeton-methyl and 2 ng/g blood for the metabolite demeton-S-methylsulfon. This method was applied for the analysis of blood and tissue samples of a 58-year-old man who died after the intake of Metasystox R-Spezial, a product from Bayer containing oxydemeton-methyl. Because of a prolonged interval between the ingestion and death, the concentrations of oxydemeton-methyl and demeton-S-methylsulfon were very low. Although the body was already putrefied when the autopsy was performed, the chromatograms of blood and tissue samples were free of interfering peaks.