The urinary excretion of injected cyanocobalamin was studied in 30 previously untreated vitamin B(12)-deficient patients by measuring the urinary radioactivity after repeated injections of (58)Co vitamin B(12). The dose range used was 54 to 30,000 mug., each patient receiving the same dose at each injection. The results show that there is no constant trend to the excretion of greater or smaller proportions of the amount injected. It is also shown that there is a wide variation in the amounts excreted by each individual and between individual patients and that the capacity of the tissues to retain injected cyanocobalamin is very great, the amounts retained often being greatly in excess of the normal body stores. The excretion of radioactivity after parenteral infections of (58)Co vitamin B(12) was usually complete within 24 hours but exceptions to this were seen. The significance and value of the results is discussed.