Most studies show that oral ingestion of iron does not cause a positive stool guaiac reaction. However, all in vitro studies show that iron does cause a positive guaiac reaction and some in vivo studies have shown a positive stool guaiac reaction in response to oral iron. This study examines this unresolved question. Twenty-five normal volunteers were given 900 mg of ferrous sulfate a day. Two Hemoccult II and two HemoQuant tests were obtained before and during iron therapy. All Hemoccult II tests were negative before and after oral iron. Four HemoQuant tests were slightly elevated before oral iron and one was slightly elevated after oral iron. Ferrous sulfate, 300 mg, was dissolved in 1 L of water. The solution was acidic (pH = 3.9) and produced a positive Hemoccult II test. When the solution was titrated with sodium hydroxide to a pH of greater than or equal to 6.0, iron precipitated out and the mixture no longer produced a positive Hemoccult II reaction. Our data show that ferric iron (Fe3+) in solution will give a positive guaiac reaction directly and ferrous iron (Fe2+) will give a positive guaiac reaction after hydrogen peroxide is added because it oxidizes Fe2+ to Fe3+. Iron solutions are acidic and when titrated toward a neutral pH, the iron is precipitated out and the solution is then guaiac-negative. Thus, our in vivo data confirm most previous in vivo studies; furthermore, the discrepancy between in vivo and in vitro studies can be explained as a pH-dependent phenomenon of in vitro iron solutions. Oral iron should not cause a positive guaiac reaction, and the HemoQuant results show no increase in occult blood loss in subjects on oral iron therapy.