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The flagellateprotozoan Ochromona8 maIhamen8i8 has a growth requirement for vitamin B12 (cobalamin), which resembles that of animals in its specificity (Coates & Ford, 1955). In particular, neither deoxyribosides nor methionine, which support the growth of lactobacilhi and vitamin B12requiring mutants of E8cherichia coli respectively, can replace vitamin B12 (Hutner, Provasoli & Filfus, 1953; Johnson, Holdsworth, Porter & Kon, 1957; A. M. White, unpublished results). As in animals, pseudovitamin B12 (adenylcobamide) and several other related analogues are also inactive (Coates & Ford, 1955; Coates & Kon, 1957). Since vitamin B2.-deficient cells of 0. malhamen8is may be readily obtained by growing cultures in the presence of suboptimum concentrations of the vitamin, this organism appeared to be particularly suitable for experiments designed to elucidate ultimately the function of vitamin B12 in mammalian metabolism. In the present work, early metabolic changes after the addition of vitamin B12 to suspensions of deficient cells were investigated on the assumption that any metabolic reaction requiring vitamin B12 as a coenzyme would be stimulated either earlier or to a greater extent than reactions affected only indirectly by lack of the vitamin. The results of this work have been reported in two communications to the Biochemical Society (White & Amstein, 1960; Arnstein & White, 1961 a).

The function of vitamin B12 in the metabolism of propionate by the protozoan Ochromonas malhamensis