Precipitin and passive hemagglutination inhibition (PHI) assays for insect material in predator stomachs are compared in terms of their underlying mechanisms, specificity, sensitivity, difficulty and expense. PHI is intrinsically much more sensitive and specific than the precipitin test and only slightly more time-consuming and expensive to perform. Entomologists are strongly urged to use the PHI assay for predator-prey studies unless it can be shown that the precipitin test or one of its agar gel derivatives is adequate for the system under study.