The ability of rinderpest virus (RPV) to replicate in vitro in adherent peripheral blood monocytes and monocyte-derived macrophages under non-stimulation conditions was investigated. When flow cytometry analysis on bovine peripheral blood mononuclear cells (PBMC) was performed, monocytic cells were seen to be targets for infection by the cell culture-attenuated RBOK vaccine strain of RPV. Viral glycoprotein (H) and nucleoprotein (N) expression in adherent blood monocytes and monocyte-derived macrophages was compared with the infection in Vero cells, in which a productive infection typical of morbilliviruses is obtained. In both cell types, the infection was m.o.i.-dependent, but the rate of viral protein accumulation was slower in monocytes/macrophages. Double-labelling experiments with monoclonal antibodies against RPV and the myeloid marker CD14 confirmed that the infected blood adherent cells were monocytes and macrophages. Productive infection of monocytes was confirmed by progeny virus titration. Permissiveness to infection was not dependent on macrophage differentiation: in vitro maturation of monocytes to macrophages before infection, did not increase the susceptibility of these cells to RPV infection. With the virulent Saudi RPV isolate, similar results were obtained, although the Saudi virus apparently had a higher rate of replication compared to the attenuated virus. These observations demonstrate clearly that bovine blood monocytes and monocyte-derived macrophages serve as hosts for a relatively slow but productive infection by rinderpest virus.