An attenuated Japanese encephalitis (JE) virus SA14-14-2 (PDK) was adapted to Vero cells, a continuous cell line that has been licensed for human vaccine production, by serial passages. The resulting virus was purified by tangential flow ultrafiltration followed by sucrose density gradient ultracentrifugation, giving 2.3 mg purified virus per liter of culture supernatant. Treatment with 0.05% formalin for 4 days at 22 degreesC completely inactivated viral infectivity while preserving its antigenicity. The purified, inactivated JE virus was formulated with alum hydroxide and administered to mice by intraperitoneal route. In terms of its ability to induce anti-JE neutralizing antibody and to protect the immunized animal against neurovirulent virus challenge, the purified, inactivated JE virus formulated with alum was equivalent to the exiting commercial mouse brain-derived vaccine (JE-VAX, Aventis Pasteur Inc.).