Infectious bovine rhinotracheitis virus replicated in cultured bovine alveolar macrophages (AM). However, yields of infectious virus were low, with maximum titers approximately 100 times that of the residual inoculum. Immunofluorescence and electron microscopic studies indicated that the majority of macrophages produced viral antigen, but after infection at a multiplicity of 0.1, only 4.1% of AM produced infectious centers. Virus-infected AM culture supernatants possessed interfering activity, probably due to interferon. Incubation of fresh AM with these fluids rendered them refractory to infection. Although AM from infectious bovine rhinotracheitis virus-immune and -susceptible donors were equally permissive and their susceptibility was unaltered by incubation with bacterial lipopolysaccharide, bovine mammary macrophages which were elicited with lipopolysaccharide became nonpermissive when further incubated for 48 h with 1 microgram of lipopolysaccharide per ml. Under these conditions, infected mammary macrophages failed to synthesize viral DNA, and there was reduced synthesis of "late" viral polypeptides.