Twenty-one monoclonal antibodies directed against the measles virus haemagglutinin have recently been obtained. These were known to fall into five groups, each defined by its effects on the biological functions of the H protein. A representative of each group was selected and examined by competitive radioimmunoassay in an attempt to deduce the relationships between antibody-binding sites on the antigen. It was found that these five antibodies formed three binding groups which recognized different but overlapping areas of the molecule. These three areas formed a series of sites which traversed the active region of the H polypeptide. A haemagglutinin-directed monoclonal antibody which displayed a haemolysin-inhibiting activity was also examined. This antibody fitted into the binding group scheme determined here and there was no additional binding site for this molecule on the F protein itself.