The amount of radioactive B12 bound by substances in kidneys of rats pre-injected with this radiovitamin was found to be approximately one-half to two-thirds of the total radioactivity by 4 independent procedures. The amounts of radioactivity in the 3 fractions obtained by differential centrifugation were also determined with only a small portion in the mitochondria. Relative percentages of the radioactivity in these 3 components remained essentially unchanged as a result of pre-injection of unlabeled B12.