Recent studies indicate that serum transferrin receptor levels are a quantitative index of tissue receptor mass. To determine whether the latter plays a role in the regulation of iron absorption, we examined the relationship between serum receptor, serum ferritin and iron absorption in healthy subjects. Using radioisotopic techniques we measured absorption of inorganic iron in 174 subjects and dietary nonhaem iron in 60 subjects. With both forms of iron, the correlation with absorption was far lower for serum receptor than for serum ferritin and was no longer significant when subjects with depleted iron stores were excluded. These results indicate that in normal subjects the iron store is the main physiological determinant of iron absorption and that in the absence of iron deficiency, tissue receptor mass, reflected by serum transferrin receptor levels, has no discernible influence.