A radioisotopic assay is reported that accurately measures an almost identical serum folate to that measured by Lactobacillus casei and can separate low, borderline, and normal folate levels. This assay utilizes a powdered milk available in any grocery. The assay takes less than 4 hr and can be run in unlimited quantity. This radioassay should simplify diagnosis of folate deficiency and be particularly useful in patients receiving antibiotics and folate antagonists, where the microbiological assay gives false low results.

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