We investigated the effect of dimyristoyltrimethylammonium propane (DMTAP) charge on area per molecule of mixed DMTAP/dimyristoylphosphatidylcholine (DMPC) bilayers in a simple model. Assuming that trimethylammonium (TAP) charge causes lateral polarization of neighboring PC molecules, we analyzed variation in area per molecule as the mole fraction of TAP increases. The theoretical predictions obtained in the present study are consistent with results of a recent molecular dynamics simulation study (Gurtovenko et al. Biophys. J. 2004, 86, 3461).