The proportion of iron liberated from twenty foods was measured after incubation in vitro with human gastric juice. 2. Results with samples of gastric juice obtained from one subject on ten occasions and from sixteen subjects on a single occasion showed good agreement, the proportions of Fe liberated covering the entire range from zero for egg to 1.0 for wine. 3. Where there was small variations between the results obtained with different samples of gastric juice attempts were made to correlate these with peptic activity, various proteins and mucoproteins, pH and total and free acid; the only factors that varied with the proportions of Fe solubilized were the various measures of acidity. 4. The results correlated highly with those of Layrisse et al. (1969) for the in vivo absorption of Fe from similar foods, although the samples used were not the same. 5. The amounts of Fe solubilized when the foods were mixed with bran, oats or egg (known to reduce the in vivo absorption of Fe from foods) were less than calculated from the sum of each, with two exceptions. 6. The results obtained with human gastric juice differed from those obtained with in vitro methods previously reported, namely treatment with dilute hydrochloric acid and double incubation with pepsin plus HCl at pH 2.5 followed by adjustment to pH 7.5. 7. The amount of Fe solubilized from soya-bean flour by gastric juice was compared with that solubilized by pepsin plus HCl; both systems were shown to be pH dependent but the amount of Fe liberated with pepsin-HCl was much greater than that known to be absorbed by man in invo. 8. It is suggested that incubation with human gastric juice may permit in vitro analyses of 'chemically-available' Fe in foods and thus be of value in food composition tables of terms of 'Fe equivalents' even though the amounts absorbed in vivo are subsequently influenced by other ingredients of the diet and by the Fe status of the individual.
