Cultured skin fibroblasts from 16 patients with either French or American pyruvate carboxylase (PC) deficiency were examined for their ability to incorporate 3H-biotin into proteins. Cell extracts were also examined for the presence of biotin-containing proteins with 35S-streptavidin, immunoreactive protein with anti-PC antibody, and PC mRNA by Northern blotting with a PC cDNA probe. All the North American presentation patients showed a 3H-biotin protein, a streptavidin protein, and an anti-PC precipitable protein at 125 kilodaltons on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of cellular proteins. They also showed a detectable mRNA species for PC on Northern blotting. Of the French presentation patients, five showed very low or absent 3H-biotin protein, streptavidin protein, and anti-PC precipitable protein at 125 kilodaltons. Three French presentation patients showed PC protein to be present on the basis of these techniques. Similarly, five showed either very low or absent mRNA for PC on Northern blotting whereas three gave evidence of the presence of PC-specific mRNA. Thus, whereas the North American presentation of PC deficiency is associated with the presence of a mature biotin containing protein of the correct molecular weight, the French presentation may, in some (but not in all) cases, have both absent PC protein and absent PC mRNA.