Summary A comparison of the lactate dehydrogenase (LDH) isoenzyme pattern found in the red cells from patients with megaloblastic anaemia, with that of normal human red cells, and the red cells from patients with iron deficiency and haemolytic anaemias, has shown that megaloblastic anaemia, whether due to vitamin B12 deficiency, folate deficiency, or a mixed deficiency of both vitamins, causes a characteristic reversal in distribution of the faster moving isoenzymes LDH‐1 and LDH‐2. In normal red cells and those present in other anaemias, LDH‐2 activity is always greater than LDH‐1 activity, but in all cases of megaloblastic anaemias examined, LDH‐1 was consistently greater than LDH‐2 activity. This reversed LDH isoenzyme pattern is also present in the serum in megaloblastic anaemia, and is almost certainly derived from intramedullary destruction of megaloblastic precursor cells, with very little contribution from the circulatory red cells. It is well known that a feature of megaloblastic anaemia is a raised serum LDH activity, and it has been shown that a simple chloroform inhibition test made on either serum showing a raised serum LDH of over 900 units, or on the red cells, will give 2 good indication that this reversed pattern is present.

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