The total folic acid activity of the serum in health and in various diseases of man has been extensively discussed in the medical literature. These reports have almost completely neglected the possibility of folic acid activity being bound to the serum proteins. However, Toennies, Usdin, and Phillips (1956) found in their studies that the plasma factor was present in the Cohn fractions IV + V of the plasma. It was also present in the glycoproteins, lipoproteins, mucoproteins, and other special proteins. Attention was also focused on the part played by subfraction I V-4. Condit and Grob (1958) found that the electrophoretic mobility of folic acid activity was greater than that of albumin. The same result was reported later by Elsborg (1972). Johns, Sperti, and Burgen (1961) considered there was little or no free folic acid activity in the fasting plasma. Injected tritiumlabelled folic acid activity in plasma was 64 % protein-bound. Neal and Williams (1965) added tritiated folic acid to rat serum and studied its behaviour in electrophoresis. Folic acid activity migrated towards the anode faster than albumin and only small amounts of radioactivity were found in the protein fractions. Unbound folic acid activity was found to run in advance of the albumin, occupying a similar zone to that of natural folate. Traces of

α₂-Macroglobulin, a carrier of folic acid activity (FAA), was isolated from fresh human serum by DEAE Sephadex A-50 gel chroma-, tography. The FAA content of the maximum with α₂-macroglobulin was 136 pg/mg protein and constituted 42% of the FAA eluated with the totalproteins from the chromatography column. Theresults was corroborated by controlled retesting.

Serum proteins were divided into protein groups by DEAE-Sephadex A-50 chromatography. One of the proteins to be identified had gone towardthe cathode and contained no folic acid activity. Theother went in the electrical field toward the anode and contained all the folic acid activity applied. This proteinproved to be transferrin.

Sephadex G-200 chromatographies of the liver cell sap of rabbits, 1, 3 and 6 days after an injection of ³H-folic acid activity (FAA) into the circulation, were used in an attempt to study the binding of this labelled FAA to liver proteins. The labelled FAA was quickly accumulated in the liver, and in the cell sap it was grouped chromatographically to two maxima, which corresponded to the two maxima of microbiological FAA (L. casei). The maxima were eluated in the chromatography together with cell sap proteins. The possible coupling of FAA to proteins in the liver is discussed.