SUMMARYHybrid cells secreting monoclonal antibodies directed against the haemagglutinin (H) protein of measles virus (Edmonston) were produced by fusion of mouse myeloma cells with spleen cells derived from immunized mice. Measles antibodies secreted by these cells were tested for their ability to react with measles virus in immunoprecipitation experiments and assays of binding, neutralization, haemagglutination inhibition and haemolysin inhibition. On this basis 21 out of 75 hybridomas could be defined and divided into five functional groups with different properties. However, when tested against other measles virus strains, including those isolated from subacute sclerosing panencephalitis (SSPE) patients, normalized radioimmunoassay (RIA) binding titres showed that the extent to which a given antibody bound could vary greatly with the virus strain examined. Moreover, the biological actions within a group were found to be very heterogeneous, even when high antibody binding titres were observed. These results suggest that different measles virus strains, which are not distinguishable by polyvalent sera, do in fact possess antigenic differences. Furthermore, the functional significance of a given virus epitope may vary from strain to strain. Hybridoma antibodies were also used to demonstrate the occurrence of antigenic changes within the H polypeptide of SSPE virus during the course of a non-productive, persistent infection in vitro.