Guy's 13 is a mouse monoclonal antibody which recognizes streptococcal antigen I/II (SA I/II), a major cell surface glycoprotein of Streptococcus mutans. In a number of clinical trials, this antibody has been shown to prevent colonization in the human oral cavity. The aim of this study was to identify the SA I/II epitope recognized by Guy's 13. The data suggest that the epitope is conformational, delimited by two noncontiguous regions of the antigen: residues 45 to 457, within the N-terminal half of SA I/II, and residues 816 to 983, within the C-terminal half. In fluid-phase immunoassays a strict requirement for the simultaneous presence of both regions was demonstrated for antibody binding. Furthermore, these two regions of SA I/II were shown to have the ability to interact with each other in the absence of Guy's 13 antibody, suggesting that the normal conformation of SA I/II might be determined by the interaction of these two regions.