Solutions of cyanocobalamin in a phosphate buffer of pH 5 were irradiated with doses of 2, 5, and 10 Mrad. Concomitant with a decrease in absorption maxima following irradiation was a tendency for the peaks in the 360 to 550 m mu regions to shift towards lower wave-lentihs. Calculattons indicated that 60% of the vitamin originally present was destroyed by a 2 Mrad dose at 361 m mu and according to microbiological assay approximately 85% was inactivated at 2 Mrad. Paper electrophoresis revealed the presence of at least four compounds in the irradiated solutions which could support the growth of E. coli 113-3. (C.J.G.)