The effect of nine organic solvents and urea on hen-eggwhite lysozyme-rabbit antilysozyme precipitin reaction was studied at a ratio of the antigen to the antibody of 1:26 by weight in 70 mM sodium phosphate buffer, pH 7.0. The organic solvents used were dioxane, acetonitrile, dimethylsulfoxide, N,N-dimethylformamide, 1-propanol, propylene glycol, trifluoroethanol, ethylene glycol and glycerol. These solvents invariably caused reduction in the amount of protein precipitated during the antigen-antibody reaction. The concentration of an organic solvent, C(M), required for 50% reduction in the precipitin reaction value was determined for each organic solvent. Among the nine organic solvents, dioxane was the most potent inhibitor of the precipitin reaction. The nine organic solvents did not cause irreversible inactivation of the antigen and the antibody, and at concentrations used in this study most of them would be nondenaturing. These solvents seem to destabilize the antigen-antibody complex.