The interactions between the positively charged protein, lysozyme and the negatively charged surfactant, sodium dodecyl sulfate (SDS) in water have been investigated by determining the phase equilibria of the ternary system within the concentration range of 20 wt % protein and 20 wt % surfactant. Addition of small amounts of SDS to an aqueous lysozyme solution results in a precipitation, at protein concentrations higher than 0.1 wt %. The formation of this precipitate is due to charge neutralization of the protein (eight positive charges per protein molecule) by the surfactant. The tie lines converge toward relative concentrations corresponding to charge neutralization and the maximum amount of precipitation occurs where the charge neutralization is complete. On addition of more SDS, a complete redissolution of the precipitate occurs at about 19 SDS molecules per protein, which corresponds to the total number of positive sites along the protein chain. Between 7 and 20 wt % of protein, there is a narrow strip between the isotropic solution phase and the precipitation region, where a bluish-colored, transparent, viscous gel is formed. The molar ratio between SDS and lysozyme within the stability region of the gel phase is not more than the total number of positive sites of the protein molecule. The shape of the tie lines indicates an associative-type of interaction between the lysozyme and SDS. The phase diagram is discussed in terms of electrostatic and hydrophobic effects.