Abstract: A test composition for detecting occult blood includes an alkaline water insoluble fraction of guaiac, an organic hydroperoxide containing a tertiary hydroperoxide group, and a buffer capable of maintaining the pH of material being tested within the range of about 4.5 to about 6.8, the purified guaiac, the hydroperoxide and the buffer being present in an amount effective to produce a color change in the presence of a hemoglobin-containing material. The test composition may be combined with a test substrate, preferably a matrix of bibulous or cellulosic material to provide a test device for detecting occult blood.