Receptors for IgG Fc in many group A, C and G streptococci have been reported previously. We now describe a reaction between group A, C and G streptococci and IgG Fab-parts, not involving the antigen combining sites. The bacteria proved to bind aggregated, but not monomeric, IgG Fab-fragments, indicating that multivalent interaction was a prerequisite for the binding to occur. Most of the strains exhibiting affinity for aggregated IgG Fab could also interact with aggregated K and/or lambda light chains. The capacity of the single strains to bind aggregated IgG Fab did not correlate with presence of IgG Fc-receptor activity. Furthermore, in contrast to streptococcal Fc-receptors, the streptococcal structure reacting with Fab was highly sensitive to trypsin as well as phosphatase treatment. The binding of aggregated lambda-chains to each of two strains tested was inhibited by liquoid and aggregated human albumin, suggesting that lipoteichoic acid and/or M protein are responsible for the binding of IgG Fab-fragments. A pronounced binding of aggregated IgG Fab, but not light chains, to Staphylococcus aureus, strain Cowan I, was also found. Various other bacterial species were tested for binding of the aggregates, with negative results.