A 63-day study was conducted with young men confined to a metabolic unit to study the effects of a alpha-cellulose and phytate on zinc absorption. A liquid formula diet was used throughout the experiment. During each of three dietary treatments, (basal diet, basal diet + alpha-cellulose, or basal diet + phytate), 67Zn, a stable isotope of zinc, was added to the diets to measure zinc absorption. Zinc absorption was determined by fecal monitoring of 67Zn, measured by thermal ionization mass spectrometry. Analytical precision of all measurements was within 1%. Average zinc absorption was 34.0 +/- 6.2% from the basal diet and 33.8 +/- 2.9% from the diet with 0.5 g alpha-cellulose per kg body weight added. Zinc absorption fell to 17.5 +/- 2.5% when 2.34 g of phytate as sodium phytate were added to the basal diet. The phytate/zinc molar ratio of the diet was 15. The decrease in zinc absorption was accompanied by increased fecal zinc and decreased urinary zinc. The results suggest that phytate inhibits zinc absorption and high levels of dietary phytate could result in zinc deficiency in man.