EN
The seroneutralization test based on the inhibition of syncytial effect (SN-ST) turned out to be very useful in the serological diagnostics of enzootic bovine leukaemia. The mean coefficient value of the SN-ST for positive sera (ELISA titer from 200 to 3200) was 0.1, for doubtful (ELISA = 100) - 0.52 and for negative sera 0.94. In case of negative serum the multinuclear cells (syncytium) were very large and numerous and with 85 to 100 nuclei; for doubtful sera they were small, scarce and with 25 to 30 nuclei. No syncytia were observed with the presence of negative sera. The time of incubation of BLV with positive sera played a very important role in the process of seroneutralization. The best results of SN-ST, which corresponded to the ELISA, were found after two hours of neutralization.