The saliva interacting protein (74KSR) from Streptococcus mutans serotype f, which is immunologically related to antigen I/II from serotype c, also termed B, P1, PAc, is probably involved in the adherence process of Strep. mutans to the tooth surface. A solid-phase adherence assay showed that 38% of the binding of salivary glycoproteins to Strep. mutans OMZ175 was due to this protein. We have cloned and sequenced the 74KSR gene (sr), which produces a recombinant protein (rec195K) with a relative molecular mass of 195,000, as estimated by SDS-PAGE. The strong immunological relationship and functional identity of the 74KSR and rec195K indicate that the Mr 195K protein is probably a precursor form, post-translationally processed, of the 74KSR produced in Strep. mutans. The gene sr consists of 4667 bp and codes for a 171,177 Mr protein. Biochemical features of the protein (density in proline residues and hydrophobicity) may explain the difference observed between the SDS-PAGE estimated molecular mass of the immature protein and the one deduced from the nucleotide sequence. Intra-species hybridization experiments using three contiguous restriction fragments isolated from gene sr as probes showed that the sequence is highly similar in strains from serotypes c and e. We have also shown that a fraction of the heart specific antibodies induced in rabbits during immunization with the 74KSR or rec195K reacts predominantly with human IgG and suggest the hypothesis of antigen mimicry as an explanation for the production of anti-IgG autoantibodies. It will be of great importance to identify the cross-reactive epitopes within the molecule before considering its use in protective immunization against oral streptococci.