Particle breakage establishes limits on both size and shape in a variety of aggregation processes. The mechanisms by which breakage of isoelectrically precipitated protein aggregates occurs and models to describe the size and shear dependence of breakage rates are the subjects of this work. The analyses are based on experiments performed in a turbine-agitated vessel in which various concentrations of protein aggregates prepared under standard precipitation conditions were subjected to a range of agitation (and hence, shear) levels. The data collected were the aggregate size distributions as functions of time of exposure. Maximum aggregate size decreased with agitation level but not in the fashion predicted by past models based on maximum stable size. In contrast, a statistical model, assuming similarity of the breakage function, did give useful insights. Such analysis showed that a power law description of breakage frequency required two such functions to cover the entire distribution with a higher exponent for the larger sizes. Further, it showed that the daughter size distribution shifts from a more thorough to a more erosive mode as the aggregate volume increases. However, the relatively small reduction in size observed in these experiments did not allow conclusive proof of the similarity assumption. Mechanistic models for the kinetics of the size reduction proved the most useful. They show the expected increase in rate as the shear rate increases. The total extent of breakage depends on the solids concentration in the suspension. This is best described as a second order process involving collision of a breakage-susceptible aggregate with another aggregate. Expressions are given both for the rate of decrease in mean size and for the rate of removal by breakage of numbers of particles from the large end of the size distribution.