Previous observations on high virulence of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and Trichoplusia ni single nucleopolyhedrovirus (TnSNPV) acting together led us to test possible synergism between these two nucleopolyhedroviruses (NPVs) on cabbage looper larvae. Because synergism between AcMNPV and the Trichoplusia ni granulovirus (TnGV) has been well established before, these two viruses were included in this study as a positive control. Each virus was assayed separately on first-instar cabbage looper and their LC50s were estimated at 2.33, 0.39 and 462 OB/mm2 diet for AcMNPV, TnSNPV and TnGV, respectively. LC50s of AcMNPV mixed with sub-lethal concentrations of TnSNPV and TnGV increased 8 and 10.7 times, respectively. Synergism between the viruses was analyzed by the ANOVA test for the LC50s, the Plackett and Hewlett's joint-action rate test, and the Tammes-Bakuniak graphic method. All three analyses corroborated the synergism between the viruses. The presence of a putative enhancin in the TnSNPV was analyzed by Southern blot hybridization, using a 1.5 kbp KpnI fragment from the TnGV vef gene as a probe. No hybridization was observed. The occurrence of a new putative synergistic factor in TnSNPV is discussed.