Serum iron, serum transferrin and transferrin saturation were studied in 253 healthy, non-anaemic children 4, 8 and 13 years old, and in 60 healthy, non-anaemic adults having serum ferritin values ->15 pg/1. One hundred and ninety-six children had serum ferritin values >-15 gg/1 (i.e. replete iron stores), 35 had intermediate ferritin values from 10-14 gg/1 and 22 had ferritin values < 10 gg/1 (i.e. depleted iron stores). Iron replete children showed a gradual rise in serum iron and transferrin saturation values with age. Serum iron and transferrin saturation values were lower (P < 0.001, P < 0.0001) and transferrin values higher (P < 0.0001) in iron replete children compared to adults. Iron replete children had a 2.5 centile transferrin saturation value of 5%; 19.9% of these children had saturation values < 15% and 8.2% had values < 10%. In iron depleted children a transferrin saturation value < 7% yielded the highest diagnostic efficiency as regards exhausted iron stores, although with a low predictive value of a positive test. The transferrin saturation is unsuitable as a single diagnostic criterion in the evaluation of iron deficiency in children and should always be combined with other indicators of iron status.