Organic mercury has been an environmental problem for many years. In spite of the high toxicity of organic mercury compounds and their accumulation ability in living organisms, the spreading in the environment has been steadily increased by human activities. In this study, the two organic mercuries, methyl mercury cloride (MMC) and methoxyethyl mercury cloride (MO), have been tested for mutagenicity in mammalian cells by use of the Chinese hamster cell line V 79-4 with two genetic markers: the locus azafor nucleic acid synthesis and the locus ouaconcerning the cell membrane. High toxic effects were obtained by MMC and by MO, beside a certain low mutagenicity in both of the tested loci; MO being slightly more mutagenic than MMC. The very narrow dose response curves were found to lie close to the threshold values for toxicity. Since the two tested mercury compounds had lower threshold values in the mutagenicitytests with V 79 cells than in toxicity tests with four different materials (Allium.human lymphocytes, E. coliK39 (ks) and V 79 cells), the mutagenicity test may generally be the more sensitive tool to evaluate biological effects of chemical compounds.