The minimal daily adult folate requirement to sustain normality is unknown. Using a diet1containing approximately 5μg. of total folate activity for Lactobacillus casei per day, a healthy adult male developed folate deficiency of sufficient degree to produce megaloblastic anemia in 4 1/2 months.2Three weeks after starting this diet, the serum folate activity for Lactobacillus casei (L. casei) had fallen to below 3 mμg. per milliliter (normal 7 to 16 mμg. per milliliter).3The first hematologic abnormality, hypersegmentation of the nuclei of the polymorphonuclear leukocytes, was not noted until 7 weeks after starting the diet. Abnormally high formiminoglutamic aciduria after a 20 gm. histidine load did not appear until the subject had been for 3 months on the diet. In the present study, the minimal daily adult requirement for folic acid was equated with that amount of pteroylglutamic acid required to maintain normality of the earliest