The effect of three dietary fibers on zinc, copper, and magnesium utilization by adolescent boys was studied. The 21-day study was divided into a 2-day introductory nitrogen depletion period, a 3-day adjustment period, and a four 4-day randomly arranged experimental period. During the four experimental periods, the eight adolescent male subjects received a basal diet plus 14.2 g supplements of cellulose, hemicellulose, or pectin. All subjects received all experimental diets. Zinc, copper, and magnesium contents of food, urine and blood serum were measured by atomic absorption spectrophotometry. Relatively small changes in urinary excretion or in blood serum contents of these minerals were found that could be attributed to dietary fiber supplementation. Statistically significant changes in fecal mineral loss were demonstrated. While receiving the basal diet alone or plus hemicellulose, cellulose, or pectin, mean fecal zinc contents (milligrams per subject per day) were 9.64, 13.32, 10.26, and 9.63, respectively; mean fecal copper contents (milligrams per subject per day) were 3.78, 5.04, 4.07, and 3.91, respectively; and mean fecal magnesium contents (milligrams per subject per day) were 303.71, 379.20, 325.69, and 278.48, respectively.