cDNA encoding N(G),N(G)-dimethylarginine dimethylaminohydrolase from rat kidney had been cloned [Kimoto, M., Sasakawa, T., Tsuji, H., Miyatake, S., Oka, T., Nio, N. & Ogawa, T. (1997) Biochim. Biophys. Acta 1337, 6-10]. The enzyme hydrolyzes N(G),N(G)-dimethyl-L-arginine and N(G)-monomethyl-L-arginine, which are known as endogenous inhibitors for the nitric oxide-generating system. In the present study, human N(G),N(G)-dimethylarginine dimethylaminohydrolase has been purified to homogeneity from liver and characterized. The cDNA clone encoding human N(G),N(G)-dimethylarginine dimethylaminohydrolase was isolated from a human kidney lambda gt10 library using a probe prepared from a plasmid containing the entire coding region of rat N(G),N(G)-dimethylarginine dimethylaminohydrolase. Its open reading frame encoded a protein of 285 amino acids with a molecular mass of 31,121 Da. The deduced amino acid sequence exhibits 93% identity with that of rat. The cDNA was expressed as a fusion protein in Escherichia coli and the recombinant protein exhibited enzyme activity which is the same as that of natural enzyme.