Antisera were raised in rabbits against chromatographically purified preparations of pepsin and gastricsin. With these antisera the contents of pepsin and gastricsin in gastric juice were determined by rocket immunoelectrophoresis. The potential content of pepsin and gastricsin of a secondary standard of gastric mucosal extract was calibrated against the chromatographically purified enzymes. This secondary standard was used for routine analyses. The intra-assay and between-assay precision was 3-4% and 6-9%, respectively. Ten healthy volunteers underwent a standard pentagastrin test. The amounts of pepsin and gastricsin determined by rocket immunoelectrophoresis corresponded to the amounts observed by ion exchange chromatography of gastric juice. After stimulation with pentagastrin the secretion of both pepsin and gastricsin was increased about 10 times.