Material properties of lipid bilayers were studied on the mesoscopic scale using deuterium nuclear magnetic resonance spectroscopy. The fluid phase of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) was compared with DMPC containing a nonionic detergent as an additive. Order parameter profiles were obtained from the deuterium NMR spectra of DMPC having perdeuterated acyl chains (DMPC-d(54)). A reduction of the order parameters of DMPC-d(54) in the presence of the detergent octaethyleneglycol-mono-n-dodecyl ether (C12E8) was observed, consistent with an increased configurational freedom of the phospholipid acyl chains. Relaxation rates R-1Z((i)) and R-1Q((i)) were measured and spectral densities J(m)(m omega (0)) where m = 1,2 were calculated from the combined relaxation results. Profiles of the observables, i.e., order parameters, relaxation rates, and spectral densities were interpreted within the framework of a new composite membrane deformation model, which describes characteristic properties of the membrane in terms of a continuum-picture, According to this model, the influence of the nonionic detergent (C12E8) on the electroneutral DMPC membrane is to increase the membrane flexibility as manifested by the functional dependence of the R-1Z((i)) and R-1Q((i)) rates, i.e., the dependence of the spectral densities on the corresponding profiles of the orientational order parameters \S-CD((i))\. Within the theoretical framework the increased flexibility of the detergent-containing membranes corresponds to a decrease of the elastic constants for continuum (elastic) deformations of the membrane bilayer. In the case of splay fluctuations the elastic constant and the bilayer thickness are related to the macroscopic bending rigidity, which qualitatively yields a correspondence to studies of macroscopic bending fluctuations thus yielding support for the model. In general, these findings indicate a softening of the membrane bilayer by the presence of a nonionic detergent, which corresponds to a decrease of the elastic constants for continuum deformations of the membrane.