The most important neutralizing and protective antibodies against Measles virus (MeV) are directed against the hemagglutinin protein (MeV-H). To define the MeV binding domains recognized by human antibodies a set of 10 non-redundant MeV-H-specific monoclonal antibodies (mabs) was used to block their binding in a competition ELISA. Sera from both naturally infected and vaccinated individuals showed similar competition patterns. Two distinct domains were identified as the main target of human antibodies. One domain corresponded to the region of the previously described hemagglutinin noose epitope (HNE, aa 380-400) , which is recognized by hemagglutination- inhibiting, neutralizing and protective mabs. The second region is defined by a mab with strong neutralizing but weak hemagglutination-inhibiting activity. Mabs with a strong neutralizing capacity with respect to wild-type viruses seemed to displace more human antibodies than those with a weaker neutralizing activity. Human antibodies seem to react more weakly with the hemagglutinin regions that bind the CD46 and the fusion protein and more strongly with the putative CD150 binding site and the top loops of β-sheet 2 and 3 of the hemagglutinin.