The southern corn rootworm, Diabrotica undecimpunctata howardi Barber, can be a serious pest of peanut (Arachis hypogaea L.) pods. A laboratory bioassay was developed to test feeding cysteine proteinase inhibitors soyacystatin N (scN) and E-64 against southern corn rootworm reared on artificial diet to determine the effects on larval development and mortality. Soyacystatin N increased mortality of larvae and decreased larval development as measured by live weights at 4 and 7 d. Seven d weight gain for larvae in the control increased by 66%, while the 0.5, 1.0 and 2.0 mg/ml doses decreased by 50, 42, and 79%, respectively. The proteinase inhibitor proved to be a good candidate as a means of host-plant resistance for reducing larval development and increasing the chance to interrupt metabolism and increased mortality. E-64 also demonstrated significant larval mortality and decreased weight gain when fed through artificial diet. At 7 d, after feeding on the 0.50, 1.0, and 2.0 mg/ml doses, larval weights were decreased by 80, 150, and 190%, respectively, when compared to the controls. The deleterious effects of these inhibitors from these assays provides enough evidence that cultivars should be screened from the world collection to determine what lines may be available to use in the development of southern corn rootworm-resistant peanut lines.