Bovine leukaemia virus (BLV) and the human T-cell leukaemia/lymphoma viruses I and II represent a specific group of type-C RNA tumour viruses characterized by the presence between the err gene and the 3'LTR of an "x" region or LOR frame, which codes for a protein that trans-activates the transcription of the viral genome. As BLV can also infect sheep and induces pre B-cell specific tumours in these animals, we were interested in investigating whether suramin, a potent inhibitor of retrovirus-associated reverse transcriptase, may inhibit the in vivo multiplication of BLV in sheep. The sheep were infected with 4 X 10(7) leukocytes from a BLV-infected cow. The animals were maedi-visna virus-negative. Viral p24 antigen and reverse transcriptase appeared at 2 weeks and seroconversion occurred at 4 weeks after infection. Suramin was administered at 20 mg/kg/week from the 10th till the 16th week after infection. During the treatment period the expression of p24 antigen as well as the titre of anti-p24 and anti-gp51 antibodies were followed. Suramin treatment led to a significant, but transient, disappearance of p24 antigen and did not affect the titre of anti-p24 and anti-gp51 antibodies. The BLV-infected sheep may serve as a useful animal model for the investigation of retrovirus inhibitors and the evaluation of different therapeutic regimens.