Peritoneal macrophages from mice infected with herpes simplex virus type 2 (HSV-2) exhibited extrinsic antiviral resistance. When the macrophages were co-cultivated in vitro with virus-infected cells the yield of virus was reduced markedly. Activity was not present 1 to 2 days p.i., peaked at 3 to 4 days, declined by 7 days and was absent at 14 days after HSV-2 infection. The extrinsic antiviral activity was limited to the adherent peritoneal macrophage population. The macrophage antiviral activity was also dose-dependent, with approx. 10(6) macrophages (macrophage: host cell ratio of approx. 2:1) reducing virus plaques by greater than 90% and virus yield 1.5 to 3.0 log10. Comparable extrinsic antiviral activity was also exhibited by Corynebacterium parvum- or thioglycollate-elicited peritoneal macrophages. The macrophage activity was not species-specific, activity on Vero cells or syngeneic mouse embryo fibroblasts being comparable. Activity was also not virus-specific, as the active macrophages also inhibited vesicular stomatitis virus (VSV). The antiviral effects required viable macrophages; cell lysates did not inhibit virus growth.