The translational diffusion coeff. of an integral membrane protein/surfactant complex has been measured using a novel pulsed field gradient NMR method. In this new approach, the information about the localization of the mols. is temporarily stored in the form of longitudinal magnetization of isotopes with long spin-lattice relaxation times. This allows one to increase the duration of the diffusion interval by about 1 order of magnitude. Unlike std. proton NMR methods using pulsed field gradients and stimulated echoes, the new method can be applied to macromol. assemblies with diffusion coeffs. well below 10-10 m2 s-1, corresponding to masses in excess of 25 kDa in aq. soln. at room temp. The method was illustrated by application to a water-sol. complex of tOmpA, the hydrophobic transmembrane domain of bacterial outer membrane protein A, with the detergent octyl-tetraoxyethylene (C8E4; overall mass of complex .apprx.45 kDa). The diffusion coeff. was found to be D = (4.99+-0.07) * 10-11 m2 s-1, consistent with measurements by size exclusion chromatog. and by ultracentrifugation. The method has also been applied to a soln. of recombinant human tRNA3Lys, which has a mol. mass of 24 kDa, and the diffusion coeff. D = (1.05+-0.015) * 10-10 m2 s-1. [on SciFinder (R)]