We report the fate of 125I human IgG measles virus antibodies complexed to virus antigens expressed on the surfaces of HeLa cells persistently infected with measles virus. Each HeLa cell expressing viral antigens on its surface bound about 7.5 x 106 IgG molecules under saturation conditions. Three hours after 125I antiviral IgG bound to and saturated all antigenic sites on infected cells, 30% of the counts were released (85% TCA ppt), after 12 hr 60%, and after 24 hr 75% (30% TCA ppt). By linear sucrose density centrifugation about 30% of the counts released from infected cells at 3 hr sedimented faster than 7S IgG, 60% sedimented at the 7S position whereas 10% (non-TCA ppt) were found at the top of the gradient. Using specific rabbit antibody to measles virus hamagglutinin or nucleocapsid, we found that hemagglutinin, but not nucleocapsid antigen, was present in the heavy complexes. These experiments show that after anti-measles virus antibody binds to cell surface measles viral antigens (i.e.. hemagglutinin), immune complexes form and are shed into culture fluids. In addition part of immune complex is endocytosed and broken into small m.w. components. Shedding of virus antigen-antibody complexes from the cell's surface may be an important source of the circulating complexes found in virus-infected humans and animals.