Zinc absorption from human milk, cows' milk formulas, and soy formulas was studied in human adults by a radioisotope technique using 65Zn and whole body counting. Individual dietary components were investigated for effects on zinc absorption. Phytate was found to have a strong inhibitory effect on zinc absorption; addition of phytate to cows' milk formula (yielding a phytate concentration similar to that of soy formula) resulted in a decrease in zinc absorption from 31 to 16% similar to the absorption for soy formula (14%). Carbohydrate source, calcium, and zinc levels of the diet did not affect zinc absorption significantly. Iron supplementation of cows' milk formula decreased zinc absorption from 24 to 18% although this decrease was not found to be significant (p < 0.1). Absorption of zinc from a whey-adjusted cows' milk formula was higher (31%) than from a nonmodified cows' milk formula (22%). Increasing the zinc supplementation level in cows' milk formula but not in soy formula increased zinc absorption to approximate that from breast milk. It is suggested that reduction of phytate content of soy formula may be a more effective avenue of modification than increased level of zinc supplementation.