A method for the preparation of hydroxocobalamin from Co/sup 60/-labeled cyanocobalamin by photolysis in microgram amounts is described. As light sources, direct sunlight, north daylight, and tungsten-filament, or fluorescent lamps were used. After irradiation in aqueous solution, small columns of carboxymethyl (CM) cellulose (1 x 3 cm) were used to separate the neutral unabsorbed unchanged cyanocobalamin from the basic aquocobalamin (hydroxocobalae column was eluted with dilute sodium chloride, and the solution was concentrated under reduced pressure. Photolysis was faster with acid solutions; a trace of acetic acid was therefore usually added to make the solution 0.002 N (pH about 4.5). It was then necessary to remove the acid on a small column of diethylaminoethyl cellulose before fractionation on CM cellulose. Conversion to hydroxocobalamin is slow even in bright light. At 1 g (or less) per ml, conversion of over 95% of the cyanocobalamin can be ensured by exposure to direct (winter) sunshine for 4 hr, or to daylight on a dull day for 8 hr. (H.H.D.)