Interactions of pseudorabies virus (PRV) with peripheral blood mononuclear cells (PBMC) were studied. T-lymphocytes, B-lymphocytes and monocytes were selected or depleted from the PBMC fraction by means of several separation techniques. After inoculation with virulent PRV in vitro, the percentage of cells that expressed viral antigens was determined in the different subpopulations by immunofluorescence. The susceptibility of monocytes depended on the method of isolation. When plasma-coated polystyrene culture grade dishes were used, 17 to 26% of the monocytes showed expression of viral antigens. However, PRV antigens were detected in only 2% of the monocytes when the cells had first been labelled with monoclonal antibodies against a monocyte marker and subsequently separated on polystyrene bacteriological Petri disches coated with anti-mouse monoclonal antibodies. In subpopulations of unstimulated T- and B-lymphocytes only 0.4 to 0.7% of the cells were found positive by immunofluorescence. Viral antigens appeared in the cytoplasm and on the cell membrane of monocytes isolated on plasma-coated dishes starting 5 h after inoculation and the expression was found in a maximal number of monocytes, 7 to 8 h after inoculation. The intracellular and extracellular virus progeny titers obtained in monocyte cultures were low, ranging from 10(4.3) to 10(5.2) TClD50 per 10(6) inoculated monocytes. The percentage of inoculated monocytes which produced infectious PRV was 0.1% in two experiments and less than 0.01% in one experiment. The results indicate that although monocytes are the most susceptible subpopulation of unstimulated PBMC to a PRV infection in vitro, replication is clearly restricted. Only a subset of monocytes expresses viral antigens and an even smaller fraction produces infectious virus.