1. Values for total body vitamin B12 were calculated for eighteen patients by giving a tracer dose of radioactive cyanocobalamin and measuring the radioactivity and microbiological activity in liver biopsies obtained at laparotomy.
2. The values for total body vitamin B12 ranged from 960 to 5984 μg with a mean of 2528 μg. Five values were greater and thirteen less than the mean.
3. Significant correlations were found between the serum vitamin B12 and total body vitamin B12, the serum vitamin B12 and the hepatic vitamin B12 and the hepatic B12 and the total body vitamin B12.