The renal glomerular cross-reactivity of the amino-terminal region of type 1 streptococcal M protein was investigated. Antisera raised in rabbits against a synthetic peptide representing residues 1-26 and a peptide from which residues 20-22 had been omitted during synthesis were capable not only of opsonizing type 1 streptococci but also of reacting in immunofluorescence tests with human renal glomeruli. The cross-reactions were completely inhibited by the immunizing peptides. By using additional synthetic peptides in these inhibition studies, the glomerular cross-reactive epitope was localized to a tetrapeptide sequence Ile-Arg-Leu-Arg at positions 23-26. A number of synthetic M1 peptides containing the tetrapeptide sequence were inhibitory, whereas the M1 peptides lacking the sequence or unrelated tetrapeptides Arg-Gly-Asp-Ser or Arg-Gly-Phe-Ser were without effect. Furthermore, Ile-Arg-Leu-Arg affinity-purified antibodies reacted with renal glomeruli, and the reactivity was inhibited by the tetrapeptide as well as by type 1 M protein. These results indicate that a renal glomerular autoimmune epitope resides in a tetrapeptide Ile-Arg-Leu-Arg near the amino terminus of type 1 streptococcal M protein.