Serological techniques, using antibodies to identify prey proteins, have been applied to the determination of meal composition in cephalopods. This paper investigates the sensitivity of these techniques with respect to the persistence of identity of an individual prey source with time after ingestion and in the presence of other food items. Octopus vulgaris (Cuvier, 1797) were fed single species meals of crab ( Carcinus mediterraneus Czerniavsky) or hermit crab ( Dardanus arrosor Herbst or Eupagurus prideauxi Leach) and mixed meals, where crab and hermit crab were offered as parts of the same meal or as separate meals, 5–10 h apart. Ingested material was recovered from sections of the gut lumen (viz., the crop, stomach, caecum, and intestine), and in fluid and tissue sample extracts taken from the digestive gland. These samples were tested for positive reaction with antisera raised in rats to the prey species used in the feeding experiments. In vitro, the reaction between mammalian antibodies and the proteins (antigens) to which they have been raised, typically results in the formation of high molecular weight antibody/antigen complexes which precipitate under appropriate conditions. The sensitivities of two related immunoelectrophoretic techniques used to visualize these immunoprecipitates, crossed immunoelectrophoresis (CIE) and fused-rocket immunoelectrophoresis (FRIE) were assessed and compared. Digestive gland tissue extracts gave much stronger reactions than gut lumen or digestive gland fluid samples at all sampling times up to 20 h after feeding. After 10 h, meal identification was possible in 94% of cases, using digestive gland material and in a maximum of 55% of cases using gut lumen samples. In mixed meals, recognition of individual meal components was possible. FRIE proved the more sensitive of the two detection techniques but a miniaturization of CIE was successfully applied and is described.