A persistent infection of measles virus in hamster brain cell cultures was established. Hamster brain cells were transformed with a human papovavirus strain BK and infected with a wild-type measles virus in order to get the cell line persistently infected with measles virus. About 75 per cent of these M-HB/MVB-cells were measles virus-infected. The cells released only small amount of infectious virus and produced low levels of interferon-like activity into the growth medium. During the first 50 passages no large syncytia typical of a lytic measles virus infection were seen. The products of measles virus infection in the cells and in cell culture fluids were followed at two temperatures. Another cell line persistently infected with measles virus (Lu-carrier-cells, 28) was investigated in parallel. In both cell lines measles antigens were cytoplasmic, but during the observation period large amounts of measles nucleocapsid accumulated in the nuclei of the M-HB/MVB-cells. The virus-specific protein synthesis in M-HB/MVB-cells was weak and the intracellular amount of immunoreactive measles nucleocapsid was only 50 per cent (600 ng/10(5) infected cells) of the (1200 ng/10(5) cells) found in Lu-carrier-cells. Also the release of nucleocapsid was minimal in hamster brain cells. The decreased temperature had no clear-cut effect on virus-specific protein synthesis or on the release of the virus-specific products.