The ability of 17 monoclonal antibodies (MoAb) against measles virus haemagglutinin (MV-H) to bind to 10 selected MV-H-specific synthetic peptides was tested in an enzyme immunoassay (EIA). Three peptides representing residues 126-135 (close to the NH2 terminus), 309-318 (middle), and 587-596 (C-terminal) reacted with MoAb designated 48, I29, and 18, respectively. Binding of MoAb I29 to purified virus was abolished after pre-incubation with the peptide 309-318. Similarly, MoAb 48 did not bind to the virus after absorption with the peptide 126-135. Longer peptides of 19 residues from the regions reacting with the MoAb were also synthesized and tested in EIA. None of the MoAb recognized these longer peptides when the latter were bound as free peptides on solid phase. However, MoAb I29 binding to purified virus was blocked equally well by peptides 304-322 and 309-318. In contrast, peptide 121-139 absorbed the reactivity of the MoAb 48 much more weakly than the shorter peptide 126-135, suggesting that the conformation of the longer peptide in solution is different. To analyse affinities in the antigen-antibody reactions, the plates were washed with buffers of varying pH after absorption of the MoAb to MV or peptides. The MoAb I29 bound both to MV and peptide 309-318 with equal affinity, but MoAb 48 and 18 bound to the peptides 126-135 and 587-596 with lower affinity than to the virus. This study indicates that regions corresponding to amino acids 126-135, 309-318, and 587-596 define antigenic sites of the H protein.