Summary. Red‐cell glutathione peroxidase (GSH‐Px) activity has been assayed in 62 normal subjects and in 163 patients with various diseases. In nine cases of iron deficiency anaemia, the mean GSH‐Px activity per cell or per volume of cells was low; during the first 2–4 weeks of treatment with iron, red‐cell GSH‐Px increased in parallel with haemoglobin. High enzyme activities, above the normal range, were found in 11 cases of megaloblastic anaemia due to deficiency of vitamin B12; rapid and marked fall in GSH‐Px activity followed the start of vitamin B12 therapy. Abnormally high values for red‐cell GSH‐Px were found in acute myeloblastic leukaemia and in myelofibrosis whereas the enzyme activity tended to be low in chronic lymphocytic and in chronic myeloid leukaemia. In two cases of chronic myeloid leukaemia, red‐cell GSH‐Px increased to normal during the early stage of remission produced by cytotoxic therapy. Normal red‐cell GSH‐Px was found in six cases of polycythaemia vera. In patients with carcinoma, there was a wide variation in red‐cell GSH‐Px but the mean value was significantly less than normal.

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Erythrocyte superoxide dismutase, glutathione peroxidase and catalase activities have been measured in patients with untreated malignant lymphomas. Marked deficiencies of superoxide dismutase (P less than 0.001) and glutathione peroxidase (P less than 0.001) were found, whereas catalase levels were normal. There was no apparent difference in enzyme activities between the different histological types of lymphoma.