Zinc is essential for growth and development in the premature infant yet elevated levels in the diet may interfere with iron absorption. Relative concentrations of zinc/iron vary 2-4 fold in current premature formulas. Therefore, we studied erythrocyte incorporation of Fe58 in 5 premature infants (<2500 g birth weight) receiving either high (1200 ug/kg) or low(300 ug/kg) doses of oral zinc sulphate, concurrently with 300 ug/kg oral Fe58 as chloride with 10 mg/kg vitamin C. In this crossover study, each infant served as their own control and were randomized to receive either the high or low zinc doses 2 weeks apart. Zinc and iron were given at baseline and two weeks later by nasogastric tube in between a regular feed and flushed with saline. No additional vitamins or iron were given on the study day. Iron was extracted from blood samples (at baseline, 2, 4 weeks) by ether extraction and chromatography, then analysed by FAB-SIMS for 58Fe/54Fe ratios. Results indicated high zinc inhibited iron absorption as relative enrichment decreased in each infant, on average, by half (50.2 ± 43%). Sufficient iron in early feeding must take into account the levels of zinc in the diet. Supported by the Medical Research Council of Canada