Cytotioxicity of methyl bromide on cultured mammalian cells was examined. Lethal concentrations of methyl bromide on HeLa cells and primary cultured muscle, kidney and brain cells were similar (10 μg/ml). Acute toxic effect was induced by methyl bromide itself. And its cytotoxicity was reduced in the presence of glutathione. The action of methyl bromide seemed to be nonspecific protoplasm-toxic.