The modification of cigarette design parameters in a statistically based central composite experimental design framework incorporating paper permeability, filter pressure drop, filter ventilation and tobacco blend and the resulting effects in terms of chemical composition and some aspects of smoker behaviour have been described elsewhere by Case et al. The effect of varying these design parameters and the ability of the resultant mainstream smoke to potentially influence 'in-vitro' biological activity has also been evaluated using an Ames mutagenicity test and the In Vitro Micronucleus test (IVMNT). Both these tests are designed to complement each other in the detection of mutagenic agents and form part of a test strategy as set out in international guidelines. Cigarettes were smoked under ISO conditions to generate mainstream smoke condensate. Specifically, the genotoxic potential of smoke condensate was assessed by measuring the number of induced revertants in Ames strains TA98 and TA100 in the presence of an S9 metabolic activating fraction. To give an indication of direct acting mutagenic species in mammalian cells the IVMNT with a V79 cell line was used in the absence of an S9 metabolic activating fraction. Following treatment with condensate, cytokinesis blocked cells (Cytochalasin B) were scored for cytotoxicity, as measured by the proportion of binucleate cells and for micronucleus induction within the dividing cell (binucleate) population. Trends in the data suggest that modifying blend, ventilation, pressure drop and paper permeability in cigarettes design can have an impact on Ames specific activity of the smoke condensate. However the IVMNT results imply that there is little or no change in the specific activity of condensates if design parameters are altered within the limits of the experimental design.