Purpose: The objective of this study was to examine the cytotoxic effect of methyl methacrylate (MMA) monomer on 4 mammalian fibroblasts. Materials and Methods: Four cells--C3H10T1/2, L929, Balb/3T3 clone A31, and MC3T3-E1--were incubated for 6 days in 24-well microplates filled with 1 mL of Dulbecco’s Modified Eagle medium containing MMA monomer that ranged from 0 to 200 mM/L. Cytotoxicity was evaluated by direct cell number count. Results: L929 possessed the largest TC50 value (the amount of MMA that caused 50% cell death) (34 mM/L), and Balb/3T3 clone A31 had the least value (1 mM/L); C3H10T1/2 and MC3T3-E1 had intermediate values (25 mM/L and 16 mM/L, respectively). Conclusion: This study showed that the MMA monomer had dose-dependent cytotoxicity, which varied among the cells tested.