A GRANULOSIS virus (GV) and a nuclear-polyhedrosis virus (NPV) of the armyworm, Pseudaletia unipuncta, form a synergistic association in which the infection of the NPV is enhanced1. The synergistic factor responsible for the enhancement (56-fold) occurs in the protein of the inclusion body (capsule) that surrounds the particle of GV2,3. This factor is present in detectable quantities in one strain of GV (Hawaiian) but not in another (Oregonian)4,5. It has been purified by Sephadex gel filtration and seems to be a simple or conjugated protein3. We now report that the synergistic factor has properties of an enzyme and can enhance not only the NPV and GV of the armyworm but also other insect viruses. Earlier studies1,2 had shown that the synergistic factor was heat labile. It became inactive when heated at 85 °C for 10 min (ref. 1). The viral activity, on the other hand, was destroyed at 75 °C for 10 min.