Transfection of PDH E1 alpha cDNAs into human normal (3781) and PDH-deficient (4787) lymphoblast cell lines was performed to study the expression of different E1 alpha cDNAs. Transfection of normal human E1 alpha cDNA into a severely PDH-deficient cell line with 10% residual activity resulted in a fivefold increase in residual PDH complex activity. Transfection of the normal cDNA into the normal cell line did not affect the residual enzyme activity. Transfection of three known human PDH E1 alpha mutations (A875T, C787G, and a 13-bp insertion at nucleotide 981) into the normal cell line resulted in a decrease of PDH complex activity. Expression of these same mutations in the deficient cell line resulted in an increase of PDH complex activity, with the C787G mutation causing the greatest increase in enzyme activity. The increase in activity seen with A875T expressed in the mutant cell line suggested that interallelic complementation had occurred.