The precipitation of a monoclonal IgG2 crystalline cryoglobulin (WEB) is shown to be highly dependent on temperature and concentration. Below a critical concentration of 0.6 mg/mL there is no cryoprecipitation. The kinetics of the aggregation exhibits a concentration-dependent lag time. This evidence suggests that a nucleation event is important in the precipitation. Circular dichroism (CD) was used to investigate the conformational properties of the protein. At a low concentration (0.12 or 0.15 mg/mL), no detectable spectral changes in the far- and near-UV range were noted between 40 and 3 degrees C. However, at higher concentrations (1.21 mg/mL), a small and rapid CD change was observed in the 250-280-nm region at 3 degrees C. This indicates an intermolecular interaction that precedes the precipitation. Cryoprecipitation of WEB was also shown to be dependent on maintenance of intact interchain disulfide bonds. Only one or two interchain disulfides need be cleaved to abolish cryocrystallization and to significantly diminish the CD change at 3 degrees C. The evidence is consistent with the formation of an initial intermediate that involves interactions near the disulfide bonds in the hinge region of the cryoimmunoglobulin. In this model, cleavage of these disulfides prevents this interaction and abolishes cryoprecipitation.