To determine the significance of the commonly observed fall in serum vitamin B12 levels during pregnancy, serum levels of the B12 metabolites methylmalonic acid (MMA) and homocysteine (Hcy) were measured in a group of 50 pregnant patients with subnormal serum B12 (range 45-199 pg/ml) and the results compared with those of 25 pregnant controls (serum B12(208-580) pg/ml). Mean values for serum MMA and total Hcy in the subnormal B12 group were 445.4 nmol/L and 7.03 mumol/L, respectively, which were not significantly different from the mean MMA of 440.5 nmol/L and Hcy of 6.88 nmol/L in the controls. For the total group of patients, neither serum MMA nor serum Hcy levels correlated with serum B12. One-third of pregnant patients showed elevated serum MMA values, independent of B12 status. Significant elevation of serum Hcy was detected in only two patients, both with subnormal serum B12 and hematological evidence of B12 deficiency. We conclude that the usual fall in serum B12 concentration in pregnancy does not reflect B12 deficiency at the biochemical level. In establishing true B12 deficiency in pregnancy, the serum Hcy level (in the absence of folate deficiency) but not serum MMA, is of value.