Population dynamics of bovine peripheral blood leukocyte subpopulations were quantitated following a primary bovine herpesvirus-1 (BHV-1) infection. Percoll isolated peripheral blood mononuclear cell (PBMC) subpopulations were analyzed using flow cytometry (FC) and cytochemical stains. Between days two to eight post-infection (PI) there was a significant decrease in the percentage of T-cells and nonT/nonB cells which was accompanied by an increased percentage of B-cells and monocytes. These percentages were extrapolated to the number of Percoll isolated PBMC during this period. A decrease in the T-cell population was the primary cause of the observed lymphopenia and a relative increase in the percentage of B-cells. The increased percentage of monocytes was caused by an increased number of circulating monocytes. These monocytes were characterized by an increase in Fc receptor expression, a decrease in plastic and Sephadex-G10 adherence and no apparent change in the level of class II MHC antigen (Ia) expression. Serum cortisol was significantly elevated on day 2 PI and may have been responsible for both the reduction in circulating T-cells and a decrease in the in vitro viability of peripheral blood lymphocytes. The percentage of Ia positive PBMC was increased significantly on day 4 PI. However, on days 4 and 6 PI the summated percentages of monocytes and B-cells (total Ia expressing population) exceeded significantly the actual percentage of Ia positive cells. This apparent suppression of Ia expression did not coincide with the elevated serum prostaglandin E2 concentrations on days 8 and 10 PI.