1. This paper reports a survey of values for individual plasma cobalamins in normal subjects, hospital controls, and patients with vitamin B12 deficiency and diseases in which there are disturbances of B12 metabolism. The values were obtained by thin-layer chromatography and bioautography followed by photometric scanning or visual assessment.

2. Normal plasma contains methylcobalamin (the predominant component), deoxyadenosylcobalamin and hydroxocobalamin. The latter two compounds are not separable in the solvent system routinely used. In some samples there were traces of cyanocobalamin. The ratio of methylcobalamin to deoxyadenosylcobalamin plus hydroxocobalamin, normally greater than 1·0, was reduced in B12 deficiency. Many cases of untreated pernicious anaemia showed a high proportion (up to 40%) of cyanocobalamin. In folate deficiency there was no consistent change in individual cobalamins. Patients with leukaemias and liver disease showed a variety of changes, some attributable to alterations in plasma binding capacity and loss of deoxyadenosylcobalamin from the liver.

3. After oral or parenteral administration of cyanocobalamin, plasma cyanocobalamin increased and there was evidence of conversion to methylcobalamin. After parenteral administration of methylcobalamin, this compound increased in the plasma, but deoxyadenosylcobalamin plus hydroxocobalamin did not change.

4. Possible reasons for pathological changes in individual plasma cobalamins are discussed.

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