The clinical significance of the laboratory information provided by Fecatest, a new test based on a guaiac reaction for detecting occult blood in the feces, has been determined. In 93.8% of 243 patients with a lesion of the gastrointestinal tract diagnosed by endoscopy and/or radiology the test was positive; 88.5% of 148 patients in whom no lesion had been found by the same methods had a negative test. In vivo and in vitro comparison of the degree of sensitivity of Fecatest with Hematest and Haemoccult showed the following ratios: Hematest 5, Fecatest 3, Haemoccult 1. In point of sensitivity Fecatest thus comes between the other two tests investigated. It is concluded that Fecatest is a valuable tool in detecting lesions of the gastrointestinal tract associated with occult bleeding. The results show a better degree of clinical correlation between the test and the lesions in the gastrointestinal tract than with the other two. This view is further strengthened by the important practical advantages offered by Fecatest.