Stabilized preparations of heat-aggregated human immunoglobulin G (A-IgG) of restricted size were made by separating A-IgG by sucrose density gradient ultracentrifugation in the presence of bovine serum albumin (BSA). Periodic recentrifugation of stored fractions of the radiolabelled A-IgG indicated that the initial sedimentation characteristics were preserved. Pooled fractions of A-IgG stored for up to 16 months had the same functional activity as freshly prepared A-IgG of corresponding size when assessed by activation of the first component of complement and consumption of C4 and CH50 in normal human serum. It was also found that the reactivity of A-IgG in the C1q binding assay (C1Q-BA) and the conglutinin binding assay (Con-BA) was not altered by long-term storage of these A-IgG. Testing different batches of [125I]C1q and conglutinin with the same batch of stabilized A-IgG showed variations due to the instability of both [125I]C1q and conglutinin. The influence of these variations on the quantification of the levels of immune complexes in sera was reduced by using stable A-IgG as a reference. The assays were compared to determine the effect of the size of the aggregate. The C1Q-BA detected preferentially A-IgG of large size, while size had no influence in the Con-BA. These results suggest that the stability of A-IgG in BSA is such that this preparation may be used as a reliable standard for immune complex assays.