Abstract Administration of 6 intramuscular injections of 1000 µg. hydroxocobalamin every other day for 10 days enabled 15 individuals with initial vitamin B12 serum levels of 81 to 242 (mean 185 ± 44 µµg./ ml.) to maintain vitamin B12 concentration of the blood 6 months later at levels exceeding 3 times the pretreatment values. In 12 of these individuals who were studied for at least a 10-month period, vitamin B12 concentration in the serum 10 months after conclusion of injections was still double the initial pretreatment levels. This difference was highly significant statistically (p = 0.001). Of 11 patients followed for 15 months after conclusion of injections, in 7 the serum levels were still higher than before treatment. No comparable results were ever reported with cyanocobalamin therapy.