Heat denatured beta -lactoglobulin (beta -1g) in aqueous solution aggregates and eventually gels. The fraction of residual nonaggregated proteins decreases with heating time but is still detectable at long heating times after the gels are formed. Aggregated beta -1g does not have a significant NMR signal. Polysaccharide (dextran) with three different sizes was added to the system in small amounts so as not to perturb the gelation of the globular proteins. Pulsed field gradient NMR was used to determine the self-diffusion coefficient (D-s) Of residual nonaggregated protein and dextran as a function of heating time. D-s of the protein increased with heating time, whereas that of the dextran decreased. The decrease stopped when the gel was formed, and D-s remains constant at longer heating times. The results indicate that the pore size of the protein gel is fixed close to the gel point, possibly involving a process of microphase separation. The pore size decreases with increasing protein concentration, and D-s is much larger at 0.5 M NaCl than at 0.1 M NaCl.