Immunoelectroosmophoresis (IEO) on cellulose polyacetate strips was found to be a rapid, convenient, and highly sensitive procedure for detecting prey antigens (Lepidoptera: Tortricidae) in predators (Coleoptera: Carabidae). The antisera employed were induced with whole-body extracts of larvae of Choristoneura rosaceana Harris, Grapholitha molesta Busck, and Cydia pomonella L. Extracts of single larvae gave IEO reactions to a dilution of at least 1/1024 (mg fresh wt./μl diluent), when using as little as 1–3μl of the reactants. Larval antigens of G. molesta, C. pomonella, or C. rosaceana were detectable in laboratory-fed Pterostichus melanarius III. for c. 24, 48, or at least 72 h, respectively. Antigens of all prey species were detectable in Amara sp. and Harpalus affinis Schr. for at least 72 h.All antisera also reacted to some extent with 10–15 other Lepidoptera species and reacted slightly with extracts from starved carabids. Manipulations with the antisera (e.g. dilution, cross-absorption, and (or) antibody selection) eliminated reactions with carabids and species of Lasiocampidae, Gracillariidae, and Geometridae, but not with all species of Tortricidae or Noctuidae. The broad antigenic similarities observed among Lepidoptera appear to preclude reliable use of immunoprecipitin procedures for assessing natural predation on specific prey, unless specific antisera can be produced.