In the present study a shared idiotope was found among antibodies against a previously defined linear B-cell epitope of Semliki Forest virus (SFV). The synthetic B-cell epitope, located at amino acid positions 240 to 255 of the E2 membrane protein, was linked to an H-2d-restricted T-helper cell epitope of either SFV or influenza virus. Colinearly synthesized peptides of T-B polarity mixed with adjuvant were used to immunize BALB/c (H-2d) mice. After one booster immunization with either chimaeric peptide high serum antibody titers were measured against both synthetic peptide (240-255) and glutaraldehyde-fixed SFV-infected L cells. Against the synthetic peptide (240-255) a variety of monoclonal antibodies (MAbs) were produced that differed in reactivity with SFV, varied in heavy chain family, isotype, isoelectric point, and idiotype. Against one of the antipeptide MAbs (I02), that strongly reacted with SFV-infected L cells, an antiidiotypic MAb (ab2MAb), designated I02A3, was produced that could be inhibited in its binding to MAb I02 by the synthetic B-cell epitope. Therefore it was concluded that ab2 MAb I02A3 recognizes an idiotope closely associated with the antigen combining site of antipeptide MAb I02. This idiotope was definitively shared by two out of 15 antipeptide MAbs and by SFV-reactive antibodies present in both antipeptide sera and SFV-immune sera.