Nickel is a carcinogen in nickel refinery workers. Few chromosome studies have been performed on nickel toxicity. Therefore, this study was performed to investigate cytogenetic toxicity of nickel in human cultured lymphocytes by chromosome aberration, and sister chromatid exchange (SCE) which is a sensitive indicator of carcinogen and mutagen. The results indicate that nickel chloride and nickel sulfate led to a increase in SCE frequencies very significantly, although absolute value of SCE was low. In chromosome aberration, chromosome gap was increased to increment of concentration while chromosome break was not.