Lysozyme precipitation by polyacrylic acid (PAA), especially with a high molecular weight of 4 x 10(6), where the polymer bridging effect of PAA is thought to be dominant in the particle aggregation of lysozyme precipitates, was carried out to investigate the relationship between the bridging effect of PAA and the operation conditions in a semi-batch reactor. The bridging effect of PAA was found to be affected by the experimental conditions, such as the mixing intensity, PAA dosage, and ionic strength of the solution, which resulted in a variation in the particle size of the lysozyme precipitates due to a change in the aggregation mechanism. The polymer bridging of PAA was enhanced by increasing the agitation speed and PAA dosage and decreasing the ionic strength, which produced a larger particle size. However, under conditions of either poor mixing or small PAA dosage, smaller particles were formed with similar sizes, regardless of the ionic strength, as the bridging effect of PAA did not contribute to the precipitate aggregation. Based on the correlation between the concentration of the lysozyme precipitates and the mean particle size generated during the PAA feeding time, the point of change in the aggregation mechanism was identified and the precipitate concentration at which the mechanism conversion occurred was shorten by increasing the mixing intensity.