Using the PCR method the proviral sequence of the bovine leukaemia virus was detected in the peripheral blood leukocytes of cattle from an infected herd. BLV antibodies were determined by the ELISA and AGID tests. PCR amplification was performed with one set of 20-mer oli- gonucleotyde primers that should produce a 364 bp fragment of BLV-DNA located in the gag gene region. The reaction products were analyzed in 1 A°/< agarose, blatted to Hybond-N filter by the Southern method and probed with an 8.3 kb Sac I fragment of BLV, labelled with di- goxigenin -dVTP. In 23 animals examined by PCR the presence of proviral DNA was found in all serologically positive animals and also in three serologically negative animals. BLV-DNA was not detected in only one seropositive individual. The presented findings indicate that the determination of the proviral DNA of BLV using the PCR method is more sensitive than serological tests and it should be beneficial for the diagnosis of cattle infection with BLV.