Synthetic peptides were generated, corresponding to SU5 domain of envelope glycoprotein of Italian SRLV isolates It-561 and It-Pi1, belonging respectively to MVV- and CAEV-like genotypes. The peptides, encompassing an N-terminal variable and a C-terminal conserved antibody-binding site, were used in an ELISA assay to analyse the sera of two groups of sheep experimentally infected with these isolates. The kinetics and specificity of the humoral response to the homologous and heterologous antigen and the affinity maturation of the sera were evaluated. Seroconversion occurred between week 3 and 8. The response to SU5 antigen was mostly type-specific. The few broadly reacting sera may reflect the production of antibodies directed to the SU5 constant antibody-binding site. All sera underwent with time avidity maturation, resulting in the appearance of high affinity antibodies. This study suggests constant monitoring of the circulating viral variants to develop a panel of diagnostic peptides representative of local genotypes.