Folic acid deficiency with the picture of a megaloblastic bone marrow may develop in haemolytic anaemia, and, on the other hand, both vitamin B12 and folic acid deficiency may produce signs of haemolysis. As the correct interpretation of a positive antiglobulin reaction associated with megaloblastic erythropoiesis is particularly important, the effect of deficiency of vitamin B12 and folic acid on the results of the test was investigated in 32 patients with vitamin B12 or folic acid deficiency and a positive antiglobulin reaction was obtained in ten. There was no correlation between the result of the test and the degree of anaemia, and there was no significant difference between the incidence of positive results associated with deficiency of vitamin B12 or folic acid. In determining the significance of a positive result, the time interval before agglutination occurs is sometimes of greater value than the strength of the reaction or the result of the gamma globulin neutralization test.