To understand the effects of surfactants on membrane properties, constant pressure and temperature (NPT) molecular dynamics (MD) simulations have been performed on the fully hydrated liquid crystalline lamellar phase (C,) of pure dimyristoylphosphatidylcholine (DMPC) at 30 degreesC and its mixture with a 6.6% mol fraction of sodium dodecyl sulfate (SDS) surfactant. The presence of the surfactant causes a decrease in the area per lipid molecule (A) accompanied by an increase in lamellar d-spacing. We identify a strong interaction between the surfactant headgroup and the lipid zwitterionic phosphocholine (PC) group, which leads to a significant change in the orientation of the P- --> N+ headgroup dipole toward the bilayer interior. The surfactant hydrocarbon chain is more ordered in the membrane as compared to the pure SDS bilayer phase. These findings should be amenable to experimental verification.