Investigations were carried out on 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a potent direct-acting bacterial mutagen found in chlorinated tap water, for the potential to induce 6-thioguanine (6-TG) resistant mutations in cultured Chinese hamster V79 cells. The mutagenicity of MX was manifested when cells were treated with MX in Hanks balanced salt solution, but not in serum-free Eagle's minimal essential medium. A 12-fold higher mutation frequency over control was obtained at 12.5 micrograms/ml MX. In addition, the inhibitory effect of MX on metabolic cooperation between cocultivated 6-TG- sensitive (6-TGs) and -resistant (6-TGr) V79 cells was investigated. Recovery of 6-TGr cells significantly increased at 0.8-2.0 micrograms/ml MX due to impaired intercellular communication with 6-TGs cells. MX is believed to exert tumor-promoting and mutagenic activity in mammalian cells.