Persistent infection with the Edmonston strain of measles virus (MV) has been established in IC-21 cells, an immortalized murine macrophage cell line. Persistence was established immediately without syncytia formation or cytopathic effects. MV was expressed in the majority of the cells as evidenced by immunofluorescence microscopy, flow cytometry, infectious centers assays, and limiting dilution analysis. Hemagglutinin (H) and phosphoprotein expressed in persistently infected IC-21 cells had retarded migration in SDS-PAGE gels when compared to these proteins expressed in Vero cells. H protein differences were also found between freshly infected IC-21 cells and persistently infected IC-21 cells passaged for over 2 years. Six sublines of IC-21 cells, infected at different times, have maintained these characteristics for 2 years of passage. During this time period the intensity of immunofluorescence and the number of infectious virus particles recoverable fluctuated in five of the six cell lines. In one cell line virus expression remained at a consistent high level. The ability to establish a persistent MV infection in murine macrophages allows studies using a cell important in disseminating the infection. It facilitates experiments on immunological aspects of viral immunity by enabling cell mixing experiments with histocompatible cell populations and by making available the wide array of cellular and humoral reagents in the mouse.