Persistent states of measles virus infection have been established in HeLa cells by using Edmonston strain virus and two types of measles virus vaccine (M-VAC and Schwarz). The absolute amount of surface viral antigens expressed on these cells infected separately with the three viruses has been assessed by a newly developed method which employs [125I]-labelled Fab fragments of immunoglobulin G (IgG) from immune human sera. This method was used to determine the level of viral antigenic expression on acutely infected HeLa cells harvested at a time when 95 to 100% of cells could be lysed by antiviral antibody and complement. From our data, more than 1 X 10(6) antibody molecules must bind to each cell infected with measles virus before complement dependent lysis can occur in a homologous test system. Persistently infected cells bind 2 to 3 times less antibody than acutely infected cells and correspondingly exhibit less susceptibility to humorally-mediated immune lysis.