Mouse hybridoma antibodies against 5 different structural components of measles virus were used in immune fluorescence tests to characterize the appearance of viral antigens in productively infected cell cultures. The antibodies employed in the tests reacted specifically with the hemagglutinin (H, 79K), polymerase (P, 72K), nucleocapsid (NP, 60K) hemolysin-fusion factor (F, 41+20K) and matrix (M, 36K) proteins. Syncytia formed in lytically infected cultures and single isolated cells in persistenly infected cell cultures were both examined. Antibodies against NP and P proteins stained cytoplasmic inclusions varying in size from small dots to more confluent masses, frequently in a perinuclear position. Nuclei of infected cells contained exclusively NP antigen. Antibodies to envelope components--H, F and M--stained cytoplasmic membrane structures and also gave a granular cytoplasmic staining, especially in syncytia. Although all persistently infected cells produced NP antigen and the associated P component, they had a restricted capacity to produce demonstrable amount of envelope antigens. The occurrence of cells containing envelope antigens varied between about 50 and 5 per cent with H and F antigens giving the highest and lowest frequence values, respectively. It is proposed that a restricted capacity of cells to produce biologically active fusion protein is a prerequisite for maintaining a persistent infection in actively dividing cells in vitro.