The control of latency and reactivation of herpesvirus (HSV) infection is poorly understood. The activation of latent HSV is usually associated with a local or systemic rise in prostaglandins (PGs). It is possible that PGs may act indirectly by suppressing the inhibitory effect of interferon (IFN) on HSV replication. IFN has also been shown to decrease the number of herpetic recurrences and to speed up the healing of lesions. We investigated the effect of indomethacin (IND: a non-steroid anti-inflammatory drug which inhibits PGE2), PGE2, IFN-alpha and various combinations thereof on HSV-1 replication in established human lymphoid cells: Raji and Raji-HSV (a persistently infected subpopulation of Raji cells which continuously produces HSV-1 particles). We found that, in contrast to exogenous PGE2, IND suppressed HSV-1 replication in both cell lines. Attempts to overcome the inhibitory effect of IND by addition of PGE2 were unsuccessful. IFN also inhibited HSV-1 replication when a low multiplicity of infection was used. Moreover, the inhibition of HSV-1 multiplication was more marked in cultures treated with IFN in the presence of IND. PGE2 did not decrease or reverse the protective effect of IFN. Our results also suggest that the effects of PGE2, IND and IFN on HSV-1 replication depend on the multiplicity of infection. Further, the present observations together with previously published data would indicate that the inhibitory effect of IND on HSV-1 replication is independent of cell type or origin, while the enhancing effect of PGE2 on virus growth may depend on these factors.