The percentage absorption from a 3 mg dose of ferrous iron was measured in 50 subjects with iron stores that varied over a wide range. Iron status was assessed by a number of measurements, including the haemoglobin concentration, the plasma iron concentration, the total iron-binding capacity, the plasma ferritin concentration and the concentration of non-haem iron in the bone marrow. There were good inverse correlations between the log percentage iron absorption and both the log marrow non-haem iron concentration (r -0.94; P less than 0.001) and the log plasma ferritin concentration (r -0.78; P less than 0.001). In addition, there was a positive ferritin concentration (r +0.84; P less than 0.001). These results suggest that reticuloendothelial iron stores represent an important determinant of iron absorption and that their size can be guaged from the plasma ferritin concentration.