Human interferon-αA/D (Bgl), an alpha-hybrid cloned interferon, displays activity in both human and mouse cell lines. We measured the effects of this interferon in double and triple combinations with acyclovir, vidarabine or its 5′-monophosphate against herpes simplex virus type 2 in mouse and human fibroblasts. A 75%cytopathic effect reduction assay employing a modified checkerboard technique was used. Results in human fibroblasts were compared with those obtained when recombinant human interferonα₂ was substituted for the hybrid. Combinations of the hybrid interferon and nucleoside antiviral agents evoked comparable synergistic isobolograms and fractional inhibitory concentration indices in human and mouse cells versus herpes simplex virus type 2. Similar interactions were found when human interferon-α₂ was substituted. Uninfected cells treated with the tested combinations showed no toxicity. These data suggest that combinations of recombinant human interferonα-A/D (Bgl) and nucleosides in mouse models of herpes infection deserve study.