The relationship between aging and aggregation mechanism of solid lysozyme precipitates produced by polyacrylic acid (PAA) with a high molecular weight of 4.0 x 10(6) was experimentally investigated in a semi-batch reactor. As feeding the PAA, the aggregation mechanism between lysozyme and PAA was found to be determined by the aging history, which was shifted from charge neutralization to polymer bridging with increasing the agitation level. The lysozyme precipitates generated through a high bridging effect of PAA at a high mixing speed were broken up with aging, i.e. additional agitation of the product suspension after the feeding time. In contrast, the particle size of the precipitates formed at a low mixing intensity through a poor polymer bridging increased due to aging-induced aggregation, except for the extremely long aging time. Based on the largest particle size of the precipitates after aging, the aggregate strength was clearly confirmed to be enhanced by the bridging effect. Regardless of the mixing level, the lysozyme precipitates exhibited a steady particle size when the Camp number approached about 5 x 10(5) by adjusting the aging time with the same agitation speed as that used during the PAA feeding. However, the steady particle size was not observed when the Camp number was controlled by the aging intensity for the fixed aging time. The precipitates underwent the phenomenon of infilling at a high aging intensity, which appeared to participate, along with the shear stress, in the particle fragmentation that occurred during the aging process.