Experiments were carried out to determine the effects of endogenous phytic acid in soybean seeds (Glycine max, var. "Amsoy") on the bioavailability of 59Fe to iron-depleted rats. Soybean plants were grown in nutrient solutions labeled with 59Fe supplied at either 0.4 or 1.0 ppm iron in solution. Immature and mature soybean seeds were harvested and fed to male rats as a single dose. The phytic acid content of the immature and mature seeds averaged 0.61 and 1.71% dry weight, respectively. Varying the nutrient solution iron concentration in the mature soybean seeds. When the 59Fe-labeled seeds were fed to iron-deficient rats, 59Fe from the mature seeds was more available than that from immature seeds even though the mature seeds contained approximately three times as much phytic acid. Autoclaving the mature seeds did not affect the absorption of 59Fe by rats from the seeds. Immature soybean seeds contained much more insoluble 59Fe than did mature soybean seeds. It was concluded that the directly correlated to the phytate content of the seeds; also immature soybean seeds apparently contain a factor or factors (other than phytic acid) that depresses iron availability.