The mutagenicity of particulate matter concentrated from environmental tobacco smoke (ETS) from a prototype cigarette that primarily heats tobacco was compared to that of four popular commercially available cigarettes that burn tobacco. ETS was generated by six individuals simultaneously smoking 1 cigarette each in a 20-min time period in a 45 m3 environmental chamber operated in the static mode (without ventilation). Respirable suspended particles (RSP) were collected on polytetrafluoroethylene (PTFE) filters at a flow rate of 3 LPM for 120 min. Less ETS-RSP (86-90%) was emitted by the prototype tobacco-heating cigarette than by the tobacco-burning cigarettes. RSP was extracted from the filters by sequential sonication in acetone and dichloromethane. The acetone extract was dried under nitrogen and the dichloromethane filtrate was added and then dried to obtain ETS-RSP for testing. Mutagenicity was assessed in the microsuspension modification of the Ames Salmonella/microsome assay with strains TA98 and YG1024 in the presence of 5% S9 metabolic activation. The results show that the mutagenic activity of RSP from the prototype cigarette was reduced by 75-83% on a per-mg basis when compared to the commercially available cigarettes and was reduced by 96-98% when calculated as revertants/m3 air under identical smoking conditions.