The diaphragm cell technique was used to measure the diffusion coefficient of lysozyme in buffered solutions with and without NaCl. In the absence Of salt, the pseudobinary diffusion coefficient decreases with increasing lysozyme concentration. In the presence of NaCl, the main term D-11 decreased slowly with increasing NaCl concentrations and exceeded the pseudobinary values by 4-10%. The main term D-22 did not vary significantly with NaCl concentration and was slightly less than the binary diffusion coefficient for NaCl. The cross-term D-21 increased sharply with salt concentration. The cross-term D-12 is small and decreased slowly with salt concentration. These results are consistent with the expected electrostatic interactions between lysozyme and salt in the diffusion process.