Adding 36 g of wheat fiber for 3 weeks to the metabolically controlled diets of six subjects produced a significant increase in daily fecal weight from 70.8 g +/- 6.2 SEM to 217 g +/- 12.1; serum iron also fell by 21 micrograms/100 ml +/- 2.1 SEM (P less than 0.001) during the added fiber period (measured in five subjects) as did mean corpuscular volume and mean corpuscular hemoglobin. Fecal neutral steroid concentration (measured in four subjects) fell from 31 to 17.3 mg/g dry weight (P less than 0.05) but the change in neutral steroid output and in acid steroid concentration and output was not significant. No change was seen in the serum levels of cholesterol and triglyceride.