Measles virus haemagglutinin (H), fusion (F) and matrix (M) components were purified by affinity chromatography using monoclonal antibodies coupled to CNBr-activated Sepharose. H and M proteins were purified to homogeneity as determined by polyacrylamide gel electrophoresis by a single cycle of adsorption-desorption. The corresponding purification of the F protein required two cycles of adsorption-desorption. After the second adsorption an extra wash with 1 M-guanidine-HCl was employed to remove the contaminating cellular actin. Electron microscopic examination of the purified envelope proteins showed that, at pH 6.0, the H peplomers had a truncated conical shape (width 6.5 to 4 nm, length 16 nm), and the F peplomers had a club-like shape (dimensions of the oval head 6 X 9 nm, length 15 nm). Lengths of both peplomers were measured excluding their undiscernible hydrophobic part. The M component at pH 3.0 appeared as a rounded particle (diam. 8 nm, central accumulation of contrast 1.5 nm) suggested to include four to six M polypeptides. Rabbit hyperimmune sera were prepared against all three purified envelope components. These sera reacted only with the homologous antigen in radioimmunoprecipitation assays. Both antisera against the H and F components neutralized the virus and blocked virus-specific haemolysis, but only anti-H serum inhibited haemagglutination.