In previous studies, we determined that a combination of cysteine and serine proteinase inhibitors in the diet of Tribolium castaneum larvae was synergistic in growth inhibition.  One hypothesis for this synergism was that the combination of inhibitors prevented adaptive responses of the larvae to either of the inhibitors.  To test this hypothesis, we fed cysteine and/or serine proteinase inhibitors to T. castaneum larvae, and then determined the effect on the properties of digestive proteinases in vitro.  As previously reported, cysteine proteinse activity was major and serine proteinase activity was minor in control larvae.  Dietary serine proteinase inhibitors had minimal effects on T. castaneum larvae. However, when larvae ingested cysteine proteinase inhibitors, there was a dramatic shift from primarily cysteine-type proteinases to serine proteinases in the proteinase profile of the midgut.  Moreover, a combination of cysteine and serine proteinase inhibitors in the diet prevented this shift to serine proteinase-based digestion, and there was a corresponding substantial retardation in growth.  Therefore, it appears that the synergistic effect of a combination of cysteine and serine proteinase inhibitors in the diet of T. castaneum larvae is achieved through the prevention of the adaptive response to overcome the activity of either inhibitor.