A Hg2+-resistant Pseudomonas which can utilize glycine as a sole carbon and nitrogen source was isolated from a municipal sewage treatment plant, and the toxicity of Hg2+ in, the presence of halide ions was examined. In glycine medium without halide ions, bacterial growth was scarcely influenced even at an initial Hg2+ concentration of 25 mu M. The viable cell count decreased when adsorbed Hg2+ was more than 300 mu mol . g-cell(-1). The toxicity of Hg2+ was enhanced when the halide ion concentration was such that the [HgX(2)](0) complex (X=Br- or I-) in the solution became dominant. In the presence of [HgBr2](0) complex, complete growth inhibition of unacclimatized cells was observed at 2-3 mu M Hg2+. A sudden decrease in the viable cell count was evident when adsorbed Hg2+ was more than 15 mu mol . g-cell(-1). Growth of the isolate pre-incubated in glycine medium containing 25 mu M Hg2+ (acclimatized cells) was also inhibited completely at 10 mu M Hg2+ in, the presence of Br-. The adsorption and toxicity of Hg2+ decreased with increasing halide ion concentration in which [HgBr3](-) or [HgBr4](2-) complexes exist dominantly. These results indicate that toxicity of Hg2+ was enhanced by the formation of [HgX(2)](0) complex and was not governed by the amount of Hg2+ adsorbed to the cells.