Isothermal ternary diffusion coefficients for the system lysozyme chloride (1) + NH4Cl (2) + H2O at 25 degreesC and pH 4.5 have been measured interferometrically for five mean NH4Cl concentrations, C-2 = 0.25, 0.5, 0.9, 1.2 and 1.5 M, with C-1 = 0.6 mM. The main-term diffusion coefficient (D-11)(v) varies slowly with C-2. The main-term (D-22)(v) increases with increasing NH4Cl concentration, as does the binary D-v in aqueous NH4Cl, but the (D-22)(v) values are lower in the ternary system. The cross-term (D21)v, which relates the coupled flow of NH4Cl to the protein concentration gradient, increases sharply with increasing salt concentration, and is 19 times larger than (D-22)(v) at the highest concentration. The values of (D-12)(v) are smaller than the corresponding values previously obtained for the lysozyme chloride + NaCl + H2O system over the whole range of salt concentration studied. Using equations based on the Onsager Reciprocal Relations, we have calculated the derivative of the chemical potential of lysozyme chloride with respect to the NH4Cl concentration, and have estimated the protein cation charge. Integration with respect to the NH4Cl concentration gives the dependence of the chemical potential of lysozyme chloride on NH4Cl concentration, providing information about the driving force for nucleation and crystal growth of lysozyme chloride.