Peritoneal exudate cells (PEC) of DBA/2 mice, after 7 days of in vitro preculture and consisting of virtually 100 per cent macrophages, were able to support the replication of Herpes Simplex Virus type 1 strain WAL (HSV). Using a standard medium based on Dulbecco's Modified Eagle Medium (D-MEM), no virus replication was observed in freshly isolated PEC. However a medium based on RPMI 1640 consistently yielded higher virus titres in precultured PEC than the D-MEM medium, and also allowed virus replication in freshly isolated PEC. Macrophages derived from the spleens or the bone marrow, and precultured in the same way as PEC represented a highly pure population and were permissive for infection with HSV. Titres of about 10(6) PFU HSV were observed in PEC 48 hours after infection with 10(3) or 10(6) PFU. However, whereas a complete destruction of the cell monolayer was observed 24 hours after infection with 10(6) PFU, complete cytopathogenicity in PEC infected with 10(3) PFU required at least twice this time. In the latter situation, plaque formation was observed 24 hours after infection. PEC of different strains of mice were compared. Of these, PEC of all mice that are susceptible to HSV infection in vivo replicated HSV to the same degree as PEC of DBA/2 mice, whereas PEC of resistant C57BL/6 and C3H/HeJ mice produced 1000 fold lower titres of viral progeny. Whereas the number of infectious centres were equal in PEC of DBA/2 and C57BL/6 mice, the plaques observed after infection of confluent PEC with a low MOI were considerable smaller in cells from C57BL/6 mice. Furthermore, significantly higher titres of interferon were measured in the supernatants of HSV-infected C57BL/6 macrophages than in those of DBA/2 macrophages, and the former were made fully susceptible by the in vitro addition of an anti-interferon serum.