Dietary zinc bioavailability was evaluated with the extrinsic tag approach of stable isotope enrichment technology in growing premature infants. The fractional absorption of one dose of 70Zn was determined on 16 occasions in 13 premature infants (birth weight 1135 +/- 40 g, gestational age 29.5 +/- 0.4 wk, mean +/- SE) and was found to be 64.9 +/- 2.7%. The fractional absorption of 70Zn ranged between 48 and 79% and did not appear to be influenced by postnatal age, postconceptual age, body weight, or intake of preterm human milk, fortified-preterm human milk, or premature formula. Therefore, if absorption of the 70Zn dose reflects absorption of dietary zinc, about 60% of dietary zinc is bioavailable.