The synthesis of the 53-54 ketomethylene isostere of HEL(52-61), an antigenic decapeptide, was realised by convergent Fmoc solid-phase peptide synthesis. In addition to the target peptide, an unexpected hydroxysuccinyl by-product was recovered. The two ketomethylene epimers did not exhibit any stimulating or competitive activity. Molecular modelling studies of the analogues-MHC-II I-A(k) complex suggested a departure from the canonical polyproline II conformation which could account for the absence of binding to I-A(k) molecule