AbstractThein vitronatural killer (NK) activity of peripheral blood lymphocytes (PBL) was assessed in 13 patients with genetic haemochromatosis (HC) and 27 normal subjects, using a51Cr‐release cytotoxicity assay against the target K‐562 leukaemic cell line. Mean NK function did not differ between these two groups. This conclusion differs from the reported deficit in NK activity in other diseases in which increased iron stores may occur, including alcoholic cirrhosis and β‐thalassaemia major.The effect of ferric citrate (0.1–1.0 mmol/1), normal human liver ferritin (100–10 000 μg/1) and transferrin (2 g/1) on NK activity was also assessed for both groups. In neither group was NK activity affected by any of these additives.These results suggest that peripheral blood NK function is not compromised in haemochromatosis, and that the diminished NK activity which has previously been reported in some patients with thalassaemia or alcoholic cirrhosis is due to factors other than to a direct effect of increased