Three hundred patients with dyspepsia and 120 healthy subjects were examined by the Diagnex Blue test and the augmented histamine test. The literature is critically reviewed. Previous investigations - with a single exception - are unsatisfactory in regard to the standard of reference, the selection of patients, the number of examinations, or the report of results. This investigation shows that the Diagnex Blue test is not a reliable screening test of gastric acid secretion. Thirty per cent of acid secretors have negative Diagnex Blue tests, fifty per cent of which are false. Eight per cent of achlorhydric patients have a false positive Diagnex Blue test, and further 11 per cent of all tests show borderline results, which are useless.