A DNA clone complementary to the messenger RNA encoding the human trifunctional enzyme 5,10-methylenetetrahydrofolate dehydrogenase-5,10-methenyl-tetrahydrofolate cyclohydrolase-10-formyltetrahydrofolate synthetase has been isolated from a lambda gt10 library. In vitro transcription-translation of the 3.1-kilobase cDNA clone yields a protein of 101 kDa, which is of identical size and exhibits the same immunoreactivity as the enzyme purified from human liver. A coding region of 2805 base pairs in the cDNA encodes a protein of 935 amino acids. The initiator methionine is absent from the purified enzyme and the amino-terminal 30 amino acids derived by automated sequence analysis are identical (arginine at position 18 was not identified) with that deduced from the nucleotide sequence. The amino acid sequence of the human enzyme shows extensive homology with that of the yeast enzyme, although the amino-terminal bifunctional dehydrogenase-cyclohydrolase domain is less homologous than the carboxyl-terminal synthetase domain. A region was identified which probably serves as a link between these two major domains of the human enzyme. The synthetase domain contains two regions that are homologous to consensus sequences for an ATP-binding site.