Low serum vitamin B12 levels were found in healthy young women who were taking oral contraceptive agents (OCA). The fall in serum B12 values occurred rapidly and, in some instances, serum levels were indistinguishable from other forms of vitamin B12 deficiency. Serum B12-binding proteins or tissue levels of vitamin B12 did not appear to be altered in association with the change in serum levels. Serum folic acid levels were also reduced in some women, but folate therapy had no effect on serum B12 levels. The mechanism by which oral contraceptives alter B12 metabolism is not clear and the pathophysiologic significance remains to be elucidated. Serum B12 values obtained in women taking OCA must be interpreted, however, in the light of this phenomenon.