Bovine macrophages and lymphocyte suspensions were cultivatedin vitro from peripheral blood and used to study the interaction of bovine viral diarrhea virus (BVDV) with cells from immunized and nonimmunized animals. Using a multiplicity of infection (MOI) of 38, maximum BVDV concentration was observed 4 days after infection of macrophages, representing a 150,000-fold increase in virus. As much as 96 per cent of the virus inoculum was adsorbed by the macrophages. The addition of BVDV-neutralizing antibody to preinfected macrophage cultures for a period of 3 days resulted in a masking effect but reappearance of BVDV after antibody was removed. Replication of BVDV was also observed in peripheral bovine lymphocyte suspensions and was enhanced as much as 8-fold by the addition of phytohemagglutinin (PHA) to these cells. Macrophages and lymphocytes derived from an actively immunized cow supported BVDV replication equally as well as those from nonimmunized animals.