Eighty patients with peptic ulcer were examined by the microprecipitation test according to Hoigne. Skin tests were made in 63 and the leukocytolysis test with secretin in 39 patients who were also examined for the effect of secretin on peripheral blood cellular composition, pH of gastric juice, and for therapeutic action of the hormone. Sixty-six per cent of peptic ulcer patients showed secretin antibodies and positive skin tests, whereas 59% of patients the positive leukocytolysis test with secretin. The endogenous hormone provoked a reduction in the leukocyte, lymphocyte and neutrophil counts, raised pH of gastric juice and exerted a beneficial therapeutic action. Secretin antibodies are likely to bind the hormone and give rise to secretin deficiency. Administration of secretin has a good therapeutic effect in duodenal ulcer patients.