We investigated whether determination of serum transferrin receptor (TfR) is useful for detecting iron-deficiency in patients with chronic inflammatory diseases and for differentiating between iron-deficiency anaemia and anaemia of inflammation. Using an immunofluorometric assay, serum TfR was measured in 34 anaemic patients. Of these patients, 23 had a chronic rheumatic disease, 13 with both inflammation and iron-deficiency and 10 with anaemia of inflammation only; the other 11 patients had iron-deficiency anaemia and no evidence of inflammation. Serum TfR concentrations were lower in patients with anaemia of inflammation (2.6 +/- 0.2 mg/l, mean +/- S.E.M.) than in patients with iron-deficiency anaemia (6.7 +/- 1.1 mg/l, P < 0.01) or those with both inflammation and iron deficiency (5.8 +/- 1.0 mg/l, P < 0.01). Among patients with inflammatory disease, correlations between TfR and ferritin concentrations (r = -0.62, P < 0.05) and TfR and erythropoietin concentrations (r = 0.69, P < 0.001) were observed in iron-deficient subjects only. TfR, though not superior to serum ferritin, can help to distinguish between anaemia of inflammation and iron-deficiency anaemia and to identify iron-deficiency in subjects with chronic inflammation.