The bilayer-substrate coupling in fluid bilayers of dipalmitoyl phosphatidylcholine (DPPC) on a solid support of spherical silica beads was examined by measuring the lateral diffusion of the lipids in both monolayers using a deuterium NMR relaxation technique. The results obtained at 55 degrees C show that the lipid diffusion constant in the monolayer facing the silica surface, D = 7.5 x 10(-12) m(2)/s, is slower by a factor of 2 than that in the monolayer exposed to the bulk water (D = 14 x 10(-12) m(2)/s). This indicates that the monolayer-monolayer coupling in fluid bilayers must be rather weak compared to the monolayer-substrate coupling across an ultrathin water film between the bilayer and the silica surface.