An immunochemical test for fecal occult blood has been evaluated. It has been found to be specific for human hemoglobin and to be reproducible, accurate and four times more sensitive than chemical occult blood tests. Storage of prepared slides at -20 degrees C prevented reduction in sensitivity. To determine the effect of blood from the upper gastrointestinal tract, six volunteers ingested 100 ml of their own blood. Positive chemical, but no positive immunochemical tests were produced. In 20 healthy subjects, challenge with red meat and vegetables with high peroxidase content increased the positivity rate of chemical tests but had no effect on the positivity rate of the immunochemical test. The immunochemical method for fecal occult blood has advantages over chemical testing in that it is specific for human blood and for lower gut bleeding. Its increased sensitivity should result in a high detection rate of colorectal neoplastic lesions. However, this same increased sensitivity may also reduce its effectiveness in bowel cancer screening because of positive results in patients with trivial blood loss from non-neoplastic colonic sources.