Screening for fecal occult blood has been shown to be effective in reducing the colorectal cancer mortality. Guaiac-based tests are one of the most frequently used screening tests, but their false positivity and negativity rates are quite high. Peroxidase enzyme, found in many fruits and vegetables causes a positive interference with the method. Immunochemical methods which utilize the antibodies against human hemoglobin are devoid of these interferences. In this study, we first evaluated the within-run precision of the immunochemical method and found CV as 2%. Then, to evaluate the sensitivity of the methods we added human blood at serial dilutions to obtain decreasing amounts of hemoglobin. We obtained positive results at 0.06 mg Hb/g feces in immunochemical method and at 1.3 mg Hb/g feces in guaiac method. Then 5 volunteers were kept on a diet free of meat, fish and peroxidase containing vegetables for 6 days and on 4, 5 and 6th days they took Fe++tablets at therapeutic doses. The specimens obtained on 3rd and 6th days were analyzed by both methods. 2 of the specimens obtained on the 3rd day were positive with guaiac test and all were negative with immunochemical method. The use of iron interfered in one specimen with guaiac test, but in none with immunochemical method. To evaluate the peroxidase interference, we added horse-radish peroxidase at concentrations from 0.05 mg/g feces to 5 mg/g feces. While there was no interference in immunochemical method, in guaiac method we obtained positive results in specimens containing 0.1 mg and more peroxidase per g feces.