Using slight modifications to the method of Kohn, Mollin, and Rosenbach (1961) the histidine loading test has been carried out in 210 pregnant women. Seventy per cent. of the cases showed complete haematological correlation and this rose to 87% when marrow puncture was performed. False positive results occurred in some cases showing active erythropoiesis in response to iron therapy. False negative results were noted when other complications in addition to anaemia were present, and it was felt that these might interfere with the metabolism of histidine. In some cases the histidine test anticipated the haematological change.Seventy-seven per cent. of women with multiple pregnancies showed evidence of folic acid deficiency. Only in a few cases was the test positive before the 25th week of pregnancy. These were either cases of multiple pregnancy, haemolytic anaemia, malabsorption syndrome, or women with a recent history of megaloblastic anaemia of pregnancy. The test appears to confirm the significance of white cell changes as an indication of folic acid deficiency in pregnancy.