A novel type of enzyme immunometric assay has been developed for a heptapeptide, BN 52080. This compound is a short C-terminal analogue of sorbin and is under clinical evaluation for treatment of chronic diarrhea. In this solid-phase immobilized epitope immunoassay (SPIE-IA), the peptide is first immunologically bound to polyclonal antibodies adsorbed to a solid phase and then, after covalent immobilization with glutaraldehyde, is released from the antibody paratope by NaOH. The peptide linked to the solid phase is further quantified with a tracer consisting of the same antibodies purified by affinity chromatography and coupled to acetylcholinesterase. This assay has a detection limit of 10 pg/ml and is therefore five times more sensitive than competitive enzyme immunoassay using the same antibodies and BN 52080 coupled to acetylcholinesterase as tracer. The assay is specific and allows direct measurement of peptide in human plasma after subcutaneous or intravenous administration of 200 micrograms of BN 52080 to volunteers.