Laboratory studies were performed to characterize the lepidopteran toxicity of cotton plants expressing two different toxin proteins from Bacillus thuringiensis (Bt), in order to assess insect resistance management implications of a commercial, two-toxin transgenic cotton. An independent and additive interactive effect of two Bt -endotoxins expressed by the transgenic cotton variety 15985 was demonstrated by examining the responses of Heliothis virescens (F.), Helicoverpa zea (Boddie), and Spodoptera frugiperda (J.E. Smith) larvae to field- or greenhouse-grown tissue from genetic near-isolines, which expressed Cry1A only, Cry2Ab only, or both toxins. In all cases, the Cry2Ab component was the larger contributor to total toxicity in the two-toxin isoline. Toxin-specific, quantitative enzyme-linked immunosorbent assay (ELISA) tests confirmed that the levels of each toxin in tissues of the two-toxin isoline were not statistically different (P > 0.05) from the levels found in the corresponding tissues of the respective single-toxin isoline. Resistance management considerations were discussed. Considering the additive interaction of toxins, a relatively simple insect resistance-monitoring procedure was proposed for the monitoring of commercial cotton varieties expressing both toxins.