Sodium dodecyl sulfate (SDS) micelles provide ideal mimetic media for high-resolution NMR studies of membrane proteins and proteins or peptides interacting with micellar aggregates. N-15 NMR relaxation of the backbone amides of a protein-SDS complex has been measured under different experimental conditions. The rotational diffusion time of this complex has been found highly sensitive to detergent and NaCl concentrations. A comparison with calculated rotational diffusion times of protein-free SDS micelles under the same conditions suggests that the size of both aggregates must follow a similar functional dependence on detergent/NaCl concentration.