The extracellular domain (ECD) of the follicle stimulating hormone receptor (FSHR) has been shown to be a major determinant of hormone selectivity. The N-terminal 9-30 region, the sequence of which is unique to FSHR, has been extensively studied earlier and has been proposed to be an FSHR neutralizing epitope. In this study antipeptide antibodies specific to the peptide 9-30 were generated and used for identifying a specific immunodominant region within it. Overlapping peptides corresponding to the regions 9-19, 15-25 and 20-30 were synthesized. The ability of the antipeptide antibodies to 9-30 of FSHR to bind to different peptides was checked. The results indicated that the antibodies mainly recognized the peptide 20-30 and not the other two overlapping peptides. Further, the effect of the peptide 20-30 on the binding of radiolabeled FSH to its receptor was monitored. This peptide showed FSH-binding inhibitory activity with an IC(50) value of 0.598 x 10(-4)M and was more effective than the peptide 9-30 itself. Binding kinetics revealed that the observed effect of the peptide 20-30 is due to mixed type of inhibitory mechanism. This is the smallest peptide from the rat FSHR sequence having ability to inhibit FSH binding to its receptor by more than 90%.