Macrophages from maedi-visna virus (MVV) infected sheep have been shown to have an activated phenotype from sites of lesions in vivo. Here we have looked at the direct effect of virus infection on macrophage phenotype and activity in vitro by flow cytometry. There was no significant difference in the expression of several surface markers (CD4, CD8, MHC Class I, MHC Class II, lymphocyte function associated antigen(LFA)-1 and LFA-3) on monocyte-derived macrophages (MDM) by 5 days post MVV infection. In contrast the phagocytic activity of MVV-infected MDM for the yeast Candida utilis and erythrocytes was decreased by 5 days p.i. although the surface binding of erythrocytes was not affected. Interestingly, an activated phenotype was seen on alveolar macrophages (AM) from sheep with maedi (surface expression of MHC Class I, Class II and LFA-1 was increased), but there was no difference in the binding and phagocytosis of erythrocytes by these cells. However the binding and phagocytosis of the bacterium, Pasteurella hemolytica was increased with AM from MVV-infected sheep without lesions. Similarly there was no significant difference in the phagocytic and erythrocyte rosetting activity between fresh monocytes from MVV-infected and uninfected control sheep. Therefore the phenotype of macrophages taken from sites of lesions caused by MVV does not correspond to a direct effect by the virus on these cells or to particular activities of the macrophages.