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SUMMARY A microbiological assay for serum folic acid activity using Lactobacillus casei as the test organism is described. The method uses ascorbic acid to protect the folic acid‐like activity of serum and is sensitive enough to differentiate between sera from normal and folic acid‐deficient subjects without overlap. One hundred normal sera have been assayed and fall within the range 2·7–18·5 mμg./ml. There does not appear to be a significant difference between male and female serum levels.
Davis et al. (1962) studied this question.