The reproducibility of insulin injection tests, the comparison of insulin injection and insulin infusion tests and the specificity of the insulin injection tests were investigated in 74 duodenal ulcer patients. New failures of the insulin test were detected. The criterion of BACHRACH produced more positive "insulin tests" after injection of sodium chloride than after insulin itself. Therefore it is recommended that in single cases (recurrent ulcer) insulin injection tests (HOLLANDER criterion) should be repeated and that a sodium chloride test should be performed.