1. Serum ferritin concentrations were measured in 378 Zulu children aged 1–4 years living in Nqutu, KwaZulu and in 342 children of mixed race (Cape coloureds) aged between 1 and 16 years living near Johannesburg.
2. The pattern of serum ferritin concentrations encountered in the children at different ages tended to parallel the changes in iron stores which are known to occur during development.
3. Serum ferritin concentrations showed a significant direct correlation with age, haemoglobin, serum Fe and percentage saturation of transferrin values.
4. Anaemia was most prevalent in the younger children. Of the rural and urban children aged 13–24 months 60 and 53% respectively were anaemic while only 11% of those over 24 months had haemoglobin concentrations that were below normal.
5. The prevalence of anaemia among children who had serum ferritin values below 12 μg/l was only slightly higher than that for the groups as a whole (38.8 % v. 24.3%), while the prevalence in those with values of 12 μg/l or more was only slightly less (18.2%). However, when a ferritin concentration of less than 12μg/l was associated with a percentage saturation of transferrin of less than 16%, the prevalence of anaemia was 70.0%, and only 10.2% of children in whom both values were within the normal range were anaemic.
6. Since 67% of children had either a normal serum ferritin and a normal percentage saturation of transferrin or both values below the normal range, the two measurements taken together provided a useful means of separating the children who had significant Fe deficiency from those whose Fe stores were sufficient to make the appearance of anaemia unlikely.