Passive Heymann nephritis (pHN) is an experimental rat model for human membranous glomerulopathy. In pHN, the formation of subepithelial immune deposits (ID) involves as antigenic targets the membrane glycoprotein gp330/megalin and the 44-kD receptor-associated protein (RAP). A single binding site for ID- inducing antibodies (Abs) was previously mapped to the 86 NH2-terminal amino acids of RAP (RAP1-86). To further narrow this epitope, Abs eluted from the glomeruli were immunoblotted on membranes that were loaded with overlapping synthetic peptides representing the amino acid sequence of RAP (SPOTs system). Two adjacent Ab-binding domains with the sequences PVRLAF, (amino acids 39-44) and HSD-LKIQE (amino acids 46-53), which were separated by a single L residue at amino acid 45, were detected. Rabbit Abs raised against synthetic peptides containing these domains individually (P31-44 and P46-53) failed to procedure glomerular IDs. By contrast, Abs raised against a larger composite peptide (P31-53) induced IDs within 3d that were firmly cross linked to the glomerular basement membrane. These data suggest that Ab binding in vivo depends on the conformation of the antigenic target sequence that is preserved in the synthetic peptide P31-53, which covers the entire Ab-binding domain of RAP but not in its subdomains, P31-44 and P46-53. Collectively, these results locate the sole ID-inducing epitope of RAP to amino acids 39-53.