An extraction test for stool occult blood was developed using 3,3', 5,5' tetramethylbenzidine (TMB), which gave reliable results in the presence of interfering substances, such as vitamin C, meat fibers, and vegetable enzymes. The new test was applied to 1,320 fecal specimens from hospitalized patients not on dietary restrictions. Of 189 specimens unequivocally positive by slide tests, 48 were negative by the extraction test, 42 were trace reactions, and 99 were 2+ to 4+. Extraction testing was performed on 351 of the 1,082 specimens negative by slide tests. Four of these were from patients taking supplemental vitamin C. Two of these specimens were extraction test positive ( ) and two were extraction test negative. One other specimen, from a patient not on supplemental vitamin C, was extraction test positive. Forty-nine specimens gave equivocal results, with two commercial slide tests for occult blood. By the extraction test, 26 of these were negative, 13 had trace amounts of blood, and 10 were 2+ or 4+.