The envelope glycoprotein of small ruminant lentiviruses (SRLV) is a major 
target of the humoral immune response and contains several linear B-cell 
epitopes. We amplified and sequenced the genomic segment encoding the SU5 
antigenic site of the envelope glycoprotein of several SRLV field isolates. With 
synthetic peptides based on the deduced amino acid sequences of SU5 in an 
enzyme-linked immunosorbent assay (ELISA), we have (i) proved the 
immunodominance of this region regardless of its high variability, (ii) defined 
the epitopes encompassed by SU5, (iii) illustrated the rapid and peculiar 
kinetics of seroconversion to this antigenic site, and (iv) shown the rapid and 
strong maturation of the avidity of the anti-SU5 antibody. Finally, we 
demonstrated the modular diagnostic potential of SU5 peptides. Under Swiss field 
conditions, the SU5 ELISA was shown to detect the majority of infected animals 
and, when applied in a molecular epidemiological context, to permit rapid 
phylogenetic classification of the infecting virus.