C-13 NMR spectroscopy and the pulse gradient spin-echo NMR technique (PGSE-NMR) have been used to study the structure and mobility of the complex formed by sodium dodecyl sulfate (SDS) and gelatin. Gelatin interacts with micelle-like aggregates of SDS through electrostatic attraction of cationic residues (lysine, arginine) with anionic surfactant headgroups and through hydrophobic attraction of nonpolar and aliphatic residues (leucine, isoleucine, valine, etc.) with exposed hydrocarbon tails of the surfactant. Such interactions lower the micelle diffusion coefficient (D-mic by almost an order of magnitude : D-mic = (1-2) X 10(-11) m(2)/s in 5% aqueous gelatin at 45 degrees C while D-mic = (10-20) x 10(-11) m(2)/s in water at 45 degrees C.