While according to recent investigations cyanocobalamin (CN‐B12), so far the most commonly used vitamin B12preparation, must be considered an “artificial product”, hydroxocobalamin (OH‐B12) is probably one of the forms of vitamin B12which occurs naturally in the animal organism.The main object of the present study was to elucidate the difference in retention following intramuscular injection of CN‐B12and OH‐B12, among other things with a view to the applicability of OH‐B12in the treatment of pernicious anaemia.After i. m. injection of about 1 mg CN‐B12and OH‐B12, normal subjects excreted within 24 hours about 80 per cent and about 25 per cent, respectively, in the urine. This corresponds a retention of about 20 per cent CN‐B12compared with about 75 per cent OH‐B12.The serum concentrations about 1 hour after the injection were approximately the same whether CN‐B12or OH‐B12was given. Thereafter, the concentration of CN‐B12fell far more rapidly than that of OH‐B12, so that during the subsequent 48 hours the OH‐B12concentration was 3–6 times higher than the CN‐B12concentration.Dialysis experiments showed that OH‐B12passes far more slowly through a cellophane membrane than does CN‐B12, and that OH‐B12is bound to the serum proteins in far greater quantities than is CN‐B12. The amount of bound, i. e. non‐dialysable, CN‐B12increased only slightly with increasing total concentration, while the amount of bound OH‐B12increased proportionally, making up about two‐thirds of the total concentration.These two factors – greater binding to the serum proteins and slower diffusion of non‐bound OH‐B12– reduce glomerular filtration and must be considered the main explanation why far less of injected OH‐B12than of injected CN‐B12is lost in the urine.It is concluded that owing to its excellent retention in the organism – without addition of absorption‐retarding substances – hydroxocobalamin (OH‐B12) must be particularly suited for the treatment of pernicious anaemia and other B12deficiencies, all the more so as OH‐B12is presumably a physiological vitamin B12.In maintenance