Ninety-three bacterial strains, representing 16 Gram-positive and Gram-negative species, were tested for quantitative binding of 125I-labelled monomeric and thermally aggregated human IgG. Aggregated IgG bound to all bacerial species studied, in contrast to monomeric IgG, which interacted only with S. aureus, group A, C and G streptococci, viz. bacteria possessing previously described IgG-Fc receptors. A positive correlation was observed between binding of monomeric IgG and the uptake of thermally aggregated IgG (r = 0.92). Monomeric IgG inhibited effectively the binding of monomeric IgG but only partially the uptake of aggregates. Absorption with bacteria revealed that only a fraction of aggregated IgG could interact with bacteria lacking specific IgG-Fc receptors. A human group G streptococcus strain (G-148), tested with increasing amounts of immunoglobulin, was capable of binding at least ten times as much aggregates as monomeric IgG, implying binding to separate binding sites. These data indicate that polymeric IgG produced by thermal aggregation of human polyclonal IgG can interact with bacterial surface components found in most pathogenic microorganisms. This interaction seems to be less specific than the binding to previously described IgG-Fc receptors.