Marginal maternal biotin deficiency reduces hepatic activity of biotin-dependent carboxylases and causes high rates of fetal birth defects in mice. We tested the hypothesis that the decreased carboxylase activity observed in deficient dams and their offspring is mediated by decreased abundance of biotinylated carboxylases, decreased expression of their mRNAs, or both. During gestation, CD-1 mice were fed a diet that induced biotin deficiency or a biotin-sufficient diet. On gestational d 17, gravid uteri were removed, and each live fetus was examined grossly for defects. The expected high incidence of cleft palate (83%) in offspring was observed. In maternal and fetal liver, acetyl-CoA carboxylase, pyruvate carboxylase, propionyl-CoA carboxylase, and beta-methylcrotonyl-CoA carboxylase abundances were determined by Western blotting; the content of mRNAs for most of these enzymes and holocarboxylase synthetase was determined by real-time RT-PCR. Biotin deficiency significantly reduced the abundance of the carboxylases in maternal and fetal liver; neither the content of mRNAs for the carboxylases nor holocarboxylase synthetase changed. This study provides evidence that the decrease in carboxylase activities is attributable to a decrease in the abundance of biotinylated carboxylases; further, this effect is more severe in fetuses than dams.