The effects of complement activation on the antigenic component of immune complexes have been studied, using 125I-BSA-rabbit anti-BSA-IgG complexes as models. Polyethylene glycol precipitates of 4 types of IC (those formed in native normal human serum, or NHS-containing EDTA, NHS-EDTA, and preformed soluble IC incubated in NHS or NHS-EDTA immediately after preparation) were analysed by sodium dodecyl sulphate-polyacrylamide slab gel electrophoresis. A characteristic difference in the distribution of 125I-BSA in the gel was observed between the NHS- and NHS-EDTA-treated samples. With the former type of IC, a significant part (16-23%) of the label was found in gel fractions of mol. wt. exceeding that of the antigen (80-300 kDa vs. 69 kDa), whereas with NHS-EDTA-treated IC only a minimal amount (4-5%) of the radioactivity was detected in these fractions. These findings indicate that complement activation results in the covalent binding of complement to the antigenic component of IC. The practical importance of these observations is discussed. In addition, a marked difference in precipitability was observed between IC formed in NHS and preformed IC incubated in NHS.