To develop an analytical method for methylmercury (MeHg) compounds in biological samples (hair, blood, fish) with reproducibly high recovery of MeHg using heating vaporization atomic absorption spectrometry, we examined the pretreatment process of biological samples such as solubilization, degreasing and solvent extraction of MeHg using the Magos method. Samples were solubilized with NaOH at 70°C and degreased of fat components by chloroform and hexane. Hydrobromic acid (HBr) was used to acidify the solubilized solution to form MeHgBr with high solvent transferability. The fraction containing MeHg-L-cysteine complexes in aqueous solution, which had been reverse-extracted from the toluene layer, were determined as MeHg. Recoveries of MeHgBr were ≈ 95% with little variation. Analytical values for standards materials for hair and fish using the present method agreed well with certified values.