The 79 000 mol. wt. measles virion membrane glycoprotein G has been isolated from purified measles virus. Ultracentrifugation of 2% Triton X-100-treated measles virus produced a soluble supernatant fraction containing both G and F, the other external viral membrane protein. Lentil lectin-Sepharose and Sephacryl S-300 column chromatography of this fraction gave a pure preparation of G protein. Sucrose density-gradient centrifugation and SDS-polyacrylamide gel electrophoresis revealed that G was isolated from the virion membrane in the form of a disulphide-linked dimer. Antiserum prepared against purified G reacted only with the G polypeptide of measles virus in a slab gel antibody overlay technique. The antiserum also exhibited haemagglutination inhibition, virus neutralization and haemolysis inhibition activities.