A study of the clearance of a tracer 8amount of radioactive vitamin B12 from Serum was carried out, so that information on the daily dietary requirement of vitamin B12 could be obtained by applying the occupancy principle to the data. This study was then extended in order to follow the clearance of vitamin B12 from the individual binding proteins. Information on the whole body content of the tracer was used to estimate the total body B12 stores. In addition to these kinetic studies the distribution of vitamin B12 added to serum in vitro was also examined. Tae ratio of added B12 bound to each protein was noted in different clinical conditions as well as in serum samples obtained from hospital personnel. An investigation was carried out as to whether the mass of added vitamin B12 affected the percentage bound by each protein. It was found that the results were in no way affected. A brief study of the forms of vitamin B12 carried by each transcobalamin was also made but no specificity was observed. During the course of the investigations it was observed that vitamin B12 was retained on a column of Sephadex G - 200 under conditions which had not oeen previously described. This binding was studied in detail but was found to be of importance only in special circumstances. The work was concluoed with a comparison of the results obtained with Sephadex G - 200 and with other separation materials.