In polyelectrolyte precipitation of lysozyme, the combination mechanism between lysozyme and polyacrylic acid (PAA) at various experimental conditions was investigated from the stoichiometric coefficient at equilibrium in a batch reactor. At the underdosed condition of PAA to lysozyme, the lysozyme-PAA combination was performed by the electric charge neutralization and polymer bridging between lysozyme and PAA. As the PAA dosage decreased and its molecular weight increased, the bridging effect of PAA became more significant in the lysozyme-PAA combination because the more lysozyme was captured by the polymer bridging of PAA. The stoichiometric coefficient of the lysozyme-PAA combination, which was estimated from the lysozyme separation, was affected by both the PAA dosage and its molecular weight, but the effect of agitation was not observed. The linear equations were derived from the relationship between the stoichiometric coefficient and the dosage at three different PAA molecular weights of 2.0 x 10(3), 4.5 x 10(5) and 4.0 x 10(6). and well consistent with the experimental data.