Biotin containing carboxylases in cultured human skin fibroblasts were radioactively labeled by addition of [8,9-3H]biotin to biotin-depleted cell cultures. Three major bands were visualized by fluorography after sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the fibroblast proteins. These bands corresponded to pyruvate carboxylase (Mr = 125,000), the biotin-containing subunit of methyl crotonyl-CoA carboxylase (Mr = 75,000) and the biotin-containing subunit of propionyl-CoA carboxylase (Mr = 73,000) as judged by molecular weight markers, purified carboxylase protein standards, and interaction with monospecific antisera. Four out of 5 cell lines from patients with classical pyruvate carboxylase deficiency (less than 5% of normal activity) labeled with this technique displayed a normal band in the position of pyruvate carboxylase while one cell line showed complete absence of any labeled protein in this area. These results demonstrate heterogeneity in the etiology of pyruvate carboxylase deficiency.