Fusion of washed human erythrocyte ghosts could be induced by the addition of Sendai virus after they were loaded with bovine serum albumin and resealed. Antispectrin antibody purified on a spectrin-Sepharose column and sequestered in the ghosts at 4-5 mg/ml together with the albumin was highly inhibitory for the virus-induced cell fusion, whereas Fab fragments prepared from the same antibody were without effect. The virus-induced aggregation of intramembrane particles of human erythrocytes was also inhibited by the same concentrations of the antispectrin antibody. The virus-induced agglutinations of the ghosts and release of bovine serum albumin from the ghosts (which might be caused by fusion of the viral envelope to the erythrocyte membrane) were not inhibited by the sequestered antibody. Therefore, the antibody seems to inhibit fusion at the last step--i.e., fusion between adjacent erythrocyte membranes. Similarities and differences of the mode of participation of spectrin in the virus-induced fusion and in other membrane-linked phenomena of human erythrocytes are discussed.