The mean concentrations of serum (S)-cobalamin (vitamin B12) and S-unsaturated B12 binding capacity (UBBC) were significantly decreased in 101 women (mean age: 30.4 years) taking oral contraceptives (OC) of the combination type, compared to 113 controls. OC users more frequently showed decreased concentrations of S-cobalamin (less than 200 pmol/l) than did their controls. However, the incidence of particularly low concentrations (less than 150 pmol/l) in OC users was not increased. To study a possible dose-dependent effect, 27 women (mean age: 50.5 years) given high-dose estrogen preparations (1-4 mg estrogen) were compared with 31 controls. The two groups showed no difference with regard to S-cobalamin, but the mean S- and plasma-UBBC levels were significantly decreased in the high-dose estrogen group. 12 OC users with decreased S-cobalamin (less than 200 pmol/l), 9 OC users with normal S-cobalamin and 10 controls were studied more intensively. The mean hemoglobin concentration was significantly decreased in those OC users having decreased S-cobalamin. On the contrary, the absorption and excretion of radiolabeled cobalamin and the concentrations of erythrocyte-folate, S-iron and -transferrin did not show any difference between the groups, and all results were normal, by and large. No characteristic changes in plasma volume were found. It is concluded that routine measurement of S-cobalamin in women taking OC is not justified.