A microbiological method for the determination of folic acid activity (F.A.A.) in blood, and serum with three organisms (L. casei, S. faecalis and P. cerevisiae), is described. The S. faecalis No. 9 strain employed shows a considerably higher sensitivity than the standard ATCC No. 8043 strain, whilst the specificity towards the various F.A. derivatives tested is similar. The sensitivity of the P. cerevisiae (L. citrovorum) assay is also increased by adding thymidine, which spares the requirement for folinic acid. Finally, the procedure of extraction of the F.A.A. from blood has been modified to assure release and stabilization of the compounds assayed.

F.A.A. in blood and serum of healthy adults and infants are given.

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© 1962 by American Society of Hematology, Inc.
1962
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