A highly technological Yel-2 hybrid cell clone was isolated. The clone produces monoclonal antibodies to protein E of the yellow fever virus (Dakar strain). The Yel-2 hybridoma was cloned by the method of limiting dilutions. By the data of indirect immunofluorescence the titre of the specific antibodies reached 1:128 in the culture fluid and 1:10240 in the ascitic fluid. The Yel-2 antibodies had no cross reactions with forest-spring encephalitis, Dengue 2 and Dengue 4 viruses. In the hemagglutination inhibition test the activity of the Yel-2 ascitic preparations was observed at a dilution of 1:50000 while the complement fixation test did not reveal it. Administration of the Yel-2 cells into the abdominal cavity of mice BALB/c induced development of ascitic tumors in 97 per cent of pristane-sensitized mice and in 90 per cent of nonsensitized mice. The hybridoma was successfully transplanted from mouse to mouse (more than 20 passages during the observation period) and maintained high constant levels of specific antibody secretion. The high ascite forming capacity of the Yel-2 hybridoma provided decreasing of the cell dose administered to the mice from 10(7) to 10(4) cells per mouse.