We have studied the effect of several structurally related mansonones on the cytotoxicity of plant and bacterial toxins in Vero and BER-40, a brefeldin Aresistant mutant of Vero cells. Mansonone-D (MD), a sesquiterpenoid orthonaphthoquinone, inhibited the cytotoxicity of ricin, modeccin, Pseudomonas toxin, and diphtheria toxin in Vero cells to different extents. The inhibition of ricin cytotoxicity was dose dependent and reversed upon removal of the drug. Protection of ricin cytotoxicity was also observed in the presence of cycloheximide, indicating that de novo protein synthesis is not required for the protective effect. Although MD inhibited the degradation and excretion of ricin, the binding and internalization of ricin was not affected. In contrast, MD strongly reduced the specific binding of diphtheria toxin in Vero cells. Fluorescence microscopic studies show that MD treatment dramatically alters the morphology of the Golgi apparatus in Vero cells. The kinetic studies reveal that the protection of ricin cytotoxicity is the consequence of decreased toxin translocation to the cytosol in MD-treated cells. The reactive ortho-quinone moiety of MD is important for the protective effect as thespesone, a para-naphthoquinone with a heterocyclic ring structure identical to that of MD, did not inhibit the cytotoxicity of toxins. Thespone, a dehydromansonone-D, lacking two hydrogens from the heterocyclic dihydrofuran ring of MD, inhibited the cytotoxicity of ricin, but was albeit less potent than MD. Neither mansonone-E nor mansonone-H with reactive orthoquinone moiety, but with a different heterocyclic structure, had any effect on the cytotoxicity of ricin indicating that the protective effect of MD is specifically related to the overall structure of the metabolite.