The synergistic enhancement of nucleopolyhedrovirus (NPV) infection by granuloviruses (GVs) is well documented; and a GV granule protein, named viral enhancin, has been identified as an active contributor to this effect. We detected the presence of two proteins with molecular mass of 93 and 108 kDa in granules of a GV isolated from Xestia c-nigrum (L.) (XecnGV) as candidates for enhancin, and we confirmed that at least the 108-kDa protein enhances the infectivity of Mamestra brassicae nucleopolyhedrovirus (MabrNPV). We tested the effect of virion-free proteins obtained from XecnGV granules (GVPs) on MabrNPV infection, and we made a comparison with an enhancing chemical, the stilbene-derived fluorescent brightener Tinopal. Bioassay was performed employing the diet contamination method, by using second instars of Mamestra brassicae (L.) (Lepidoptera: Noctuidae). The enhancing effects of GVPs (0.1 mg/g diet) and Tinopal (1 mg/g diet) were estimated to be 70.7–81.5-fold and 26.9–33.7-fold, respectively, as calculated from the LC₅₀ values of MabrNPV with or without the additives. The additives reduced the lethal time of MabrNPV-infected larvae and they caused death at a younger instar. These results suggest that GVPs can enhance MabrNPV infection as effectively as Tinopal.