Experiments on the host cell spectrum of bovine leukaemia virus (BLV), a retrovirus closely related to the human T-cell leukaemia virus (HTLV), have yielded conflicting data. Currently, BLV is known to infect B cells, whereas its ability to infect other cell types, e.g. monocytes/macrophages, is doubtful. As monocytes/macrophages may have profound effects on the diversity of the T-cell response, we studied the possibility of in vitro infection, using bovine monocytes and SV40-transformed bovine macrophages. Proviral DNA was detected by nested polymerase chain reaction (PCR) from day 1 until the end of the experiments at either day 5 or day 80, depending on the quantity of virus used for infection. In addition, the infection was associated with morphological changes in infected cells as revealed by electron microscopy. The in vitro infection did not significantly change either the expression of surface antigens (CD11b, CD32, and major histocompatibility complex (MHC) class II) or the amounts of cytokine transcripts (interleukin (IL)-1beta, tumour necrosis factor (TNF)-alpha, IL-6 and IL-12p40) with or without lipopolysaccharide (LPS) stimulation. The data suggest that BLV can infect monocytes, but the infection does not seem to influence the function or the phenotype of these cells. Infected monocytes may, however, play a role as a viral reservoir in vivo.