The anticomplementary activity of IgG can be increased up to 20-fold by pipetting during the preparation of serial dilutions for the assay of this activity. Albumin, if added to the IgG solution before the serial dilutions, completely prevents this artifactual increase in activity. Polyethylene glycol, polyvinyl pyrrolidone, methyl cellulose, gelatin and octanol are also effective stabilizers of IgG. Repeated pipetting of IgG solutions caused marked linear increase in their anticomplementary activity. The formed anticomplementary activity was due to small amounts of highly aggregated protein. The amount of activity formed depended on at least four factors: [1] the number of pipetting steps; [2] the IgG concentration; [3] the level of albumin or other stabilizers, and [4] the pH, which influences the stabilization by albumin. The anticomplementary activity of IgG is increased up to 100-fold by exposure to gas-liquid, liquid-liquid and hydrophobic solid-liquid interfaces. Albumin and polyethylene glycol prevent the activity increase during these treatments. The tendency of IgG to aggregate at interfaces and the ability of albumin and other substances to prevent the aggregation paralleled their rate of adsorption to the air-water interface. Solutions of dansyl chloride in decane emulsified in aqueous solutions of IgG and albumin specifically label the proteins at the liquid-liquid interfaces. The mechanism of stabilization can be explained by preferential adsorption of surface-active proteins and polymers.