It has recently been shown that coal fly ash collected from coal-fired plants contains dimethyl sulfate (DMS) and monomethyl sulfate (MS) at concentrations as high as 830 ppm. Both these compounds were tested in the CHO/HGPRT system, and it was found that only DMS was cytotoxic and mutagenic to CHO cells. On a molar basis, DMS is twice a mutagenic as methyl methanesulfonate (MMS). Under our treatment conditions, maximum mutation induction and cytotoxicity were obtained after approximately 1 h. The Mutagenic potency of DMS was more stable in aqueous solutions at 4 degrees C than at the ambient temperature of 22 degrees C, but was least stable in DMSO solutions at 22 degrees C. Near-ultraviolet (near-UV) light caused an approximately twofold decrease in the mutagenic and cytotoxic effects of DMS. Although DMS produced by coal combustion could be rendered innocuous by environmental agents in a short span of time, this compound could still pose a health risk to workers closely involved in coal-combustion technology.