Publisher Summary This chapter discusses experiments conducted to investigate the ability of antibody to measles virus to alter the expression of viral antigens on and within infected cells. The monoclonal antibody directed against the measles virus hemagglutinin (HA) used in these experiments bound to the surfaces of infected HeLa cells. As a result, expression of all the virus polypeptides decreased, particularly that of the phosphoprotein and membrane protein and the fusion protein. This decrease could be quantitated in densitometer tracings of autoradiograms from infected cells treated with antibody compared to those left untreated or treated with antibody against viral polypeptides not expressed on the cells surface. Modifications of measles virus antigens by specific monoclonal antibody to virus HA prevented cellular events that would otherwise have led to the lysis and elimination of infected cells. This in turn favored the survival of these cells and was associated with viral persistence.