There is limited information on the bioavailability of Fe in infant weaning foods, mainly because of the difficulties of measuring Fe utilization directly in infants. The aim of this study was to develop a safe and relatively noninvasive method for studying Fe bioavailability (measured as percent Fe incorporation into red blood cells) in infants using 54Fe, 57Fe, and 58Fe stable isotopes. Four commonly used weaning foods were selected for study, labeled extrinsically with 57Fe- or 58Fe-enriched ferrous sulfate, and fed to five female and five male 9-mo-old fasting infants, using a multiple-dosing technique. Each food was given three times, labeled with one isotope, with a fruit juice drink containing 50 mg of ascorbic acid, and three times, labeled with a different isotope, with an ascorbic acid-free drink. Fourteen days after the last test meal, a blood sample was obtained from a heel-prick, spiked with a known amount of 54Fe, digested, and purified by ion exchange; isotopic enrichment and total Fe content were measured by quadrupole thermal ionization mass spectrometry. The proportion of administered dose of isotope circulating in the blood was calculated from an estimate of blood volume. The geometric mean bioavailability (range) was 3.0% (1.2-9.5%) in a proprietary dehydrated vegetable product, 3.0% (1.1-21.2%) in Weetabix whole-wheat breakfast cereal, 3.1% (1.2-15.4%) in wholemeal bread, and 4.3% (1.7-10.3%) in baked beans. When taken with the drink containing ascorbic acid, there was a 2-fold increase in bioavailability in all foods except the vegetable meal, presumably because this was already fortified with ascorbic acid.(ABSTRACT TRUNCATED AT 250 WORDS)