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 greater than or equal to 15 micrograms/l. One hundred and ninety-six children had serum ferritin values greater than or equal to 15 micrograms/l (i.e. replete iron stores), 35 had intermediate ferritin values from 10-14 micrograms/l and 22 had ferritin values less than 10 micrograms/l (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 less than 0.001, P less than 0.0001) and transferrin values higher (P less than 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 less than 15% and 8.2% had values less than 10%. In iron depleted children a transferrin saturation value less than 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.