Aliphatic dioxirane chemical structures provide a new class of potentially valuable dental monomers. Mutagenicity results from the Ames Salmonella assay for aromatic oxirane monomers were previously shown to correlate with alkaline unwinding fluorometric DNA damage and flow cytometric cell cycle analysis results in L929 mouse fibroblast cells. Objective: To investigate the effect of small structural differences on the genotoxicity of two aliphatic dioxirane monomers in L929 mouse fibroblast cells. Method: L929 cells were incubated with subtoxic doses of Cyracureä UVR6105 and its dimethyl derivative 7-oxabicyclo- [4.1.0] heptane-3-carboxylic acid, 4-methyl-(4-methyl-7-oxabicyclo[4.1.0] hept-3-yl-methyl ester (ERL-4201) for 24 hrs at 37EC in 5% CO₂ and then DNA damage and cell cycle  effects were analyzed.  Results: Cyracureä UVR6105 and ERL-4201 were relatively nontoxic with IC₅₀ greater than 1000 mM, determined by 24-hour MTT assay. Cyracureä UVR6105 exposure resulted in significantly (p<0.05) more DNA damage than ERL-4201.  Compared to solvent control, alkaline unwinding fluorometric analysis detected 35 ±6 and 65 ±12% DNA damage for Cyracureä UVR6105 and 2 ± 4 and 52 ±10% DNA damage for ERL-4201 at 100 and 500 mM, respectively. Flow cytometry analysis indicated a cell cycle delay resulting in a significantly (p < 0.05) higher %S population of L929 cells exposed to 100 mM Cyracureä UVR6105 and 500 mM ERL-4201, compared to solvent control.  Conclusions: Dimethyl groups in ERL-4201 may decrease DNA interaction by sterically hindering rotation in the molecule, thus slight structural alterations may enhance the biocompatibility of aliphatic oxirane dental monomers. wetmorel@william.jewell.edu