Among 4 ascitic preparations of hybridomas producing monoclonal antibodies to the Japanese encephalitis virus (JE) hybridomas JE-4 and JE-19 had high multiplication potentials and high levels of reimplantation and transplantability in mice BALB/c. The possibility of using the complete Freund's adjuvant and pristan for priming the mice was shown. The use of pristan promoted a significant decrease in the periods of ascitic tumor development and an increase in accumulation of the cells in the ascitic tumor, in the volume of the ascitic fluid and the titers of the monoclonal antibodies. The serological assays revealed that the monoclonal antibodies produced by JE hybridomas did not react with the JE virus antigen in the hemagglutination inhibition test. However, they were highly active in the indirect immunofluorescence test. IgM of hybridoma JE-19 reacted with the JE virus antigen in the complement fixation test while monoclonal IgG produced by hybridoma JE-4 was active in the neutralization test when titrated in the cultures of the swine embryo kidney transplantable cells. The monoclonal JE antibodies did not react with the JE virus antigen in the hemagglutination inhibition test and did not bind to the antigen of the "Sofin" strain of the forest-spring encephalitis virus in the test performed with the indirect fluorescent antibody technique.