In developing countries, such as Pakistan, laboratories do not routinely screen for iron deficiency unless the patient presents with symptoms of anaemia. Efforts to prevent the often serious consequences of iron depletion are hampered in developing countries by the expense and impracticality of routinely screening patients using bone marrow examination. Assays for serum iron concentrations, total iron-binding capacity or haemoglobin and examinations of blood films, although more practical, cannot detect the earliest stages of iron deficiency. Serum ferritin appears to be a sensitive, early indicator of iron deficiency and can be easily and relatively inexpensively determined using an immunoassay kit. In the present study, serum ferritin levels were determined using immunoassay and compared to blood films, serum iron levels and total iron-binding capacity values in 300 apparently healthy Pakistanis. In the early stages of iron deficiency, serum ferritin appeared to be a sensitive measure of iron depletion.