Fluctuations in the expression of measles virus surface-associated antigens in persistently infected human Lu 106 cells were analysed by the use of human sera which preferentially reacted with the haemolysin or haemagglutinin component. The variations observed correlated with the proportion of cells expressing surface-antigen, as examined in population analysis by indirect immunofluorescence, a radio-labelled antiglobulin technique and a cytotoxicity assay. Microfluorometric analysis revealed no changes in antigen expression at the single-cell level. The number of cells that were positive by immunofluorescence among exponentially growing, low density cells remained relatively constant. These cells were more susceptible to cytotoxicity mediated by both antisera and complement than cells seeded at high density and kept in the stationary phase. The percentage of fluorescent cells among the latter cells gradually decreased. Thus cytotoxic susceptibility was related to the proportion of the total cell population that was antigen-bearing, rather than to variations in the expression of antigen at the single-cell level. In mitotic cells, polarization of antigen, as measured by indirect immunofluorescence of pre-fixed cells, was frequently seen. Often only one of the daughter cells expressed surface antigen. The results imply that cells in stationary phase may lose antigen from their surface, possibly by shedding, and furthermore that re-expression would demand a new cell cycle.