Serum, milk and urine specimens were taken from 15 bovine leukaemia virus (BLV)-positive and 20 BLV-negative cattle which had been determined previously to be infected or not by the use of a monoclonal enzyme-linked immunosorbent assay (ELISA). An ELISA was performed on the samples for the detection of IgG1 antibodies to the BLV surface glycoprotein, gp 51. The three types of samples had parallel optical density (OD) values apart from three urine samples which, although accepted as negative for anti-BLV antibodies, had numerically higher ODS than those of control BLV-negative animals. Therefore, detection of IgG1 antibodies against BLV in the urine of naturally infected animals could be an indication for the use of urine for diagnosis of BLV infection.