It has previously been shown that secretory immunoglobulin A (S-IgA) influences the sorption of oral streptococci to hydroxyapatite as well as to cell surfaces. The present experiments demonstrate that bacterial IgA proteases, which cleave S-IgA in the hinge region, are capable of interfering with this mechanism. This result was obtained with an IgA1 specific protease from Haemophilus influenzae and with a protease from Clostridium ramosum that cleaves IgA1 as well as IgA2 of A2m(1) allotype. The modulation of S-IgA-mediated effects by IgA proteases were studied by means of an in vitro method which permits quantitative determination of the sorption of radiolabeled oral bacteria to hydroxyapatite beads. Other authors have suggested that IgA protease-mediated effects may be explained by a strongly reduced antigen-binding capacity of released Fab alpha fragments. Here we present evidence that streptococci, after exposure to specific S-IgA and IgA protease, are coated with Fab alpha fragments.