The bovine leukaemia virus (BLV) is a retrovirus that infects mainly B lymphocytes of cattle, but proviral DNA can also be isolated from monocytes/macrophages. This study investigated the effect of BLV infection on surface antigens on freshly isolated peripheral blood monocytes and cultured monocyte-derived macrophages, with and without lipopolysaccharide (LPS) stimulation. The effect of BLV infection on phagocytic activity of CD14(+) monocytes was also assessed. The percentage of monocytes expressing the surface antigens CD11b, CD32 (Fc gamma RII), MHC class II and the surface antigen recognised by mAb DH59B were increased in BLV-positive cattle. In contrast, expression intensity of all markers was low in samples from BLV-positive cattle. CD14(+) monocytes from BLV-positive cattle showed less Fc gamma-receptor-mediated phagocytosis compared to monocytes from BLV-negative cattle. After 7 days in culture, there was evidence for shedding/downregulation of surface antigens on monocyte-derived macrophages, in particular on cells from BLV-positive cattle. LPS stimulation decreased the percentage of cells expressing the measured markers in monocyte-derived macrophages taken from BLV-negative cattle, but not in cultures derived from BLV-positive cattle. The results provide further evidence for an altered function of monocytes and macrophages in BLV-infected cattle. (C) 1998 Elsevier Science B.V.