LDH-A cDNA clones from mouse and human have been isolated and sequenced. Mouse pMLA 73 was found to contain the 393bp of coding region and 496bp 3' untranslated sequence including poly (A) tail. Four human cDNA clones consist of sequences of approximately 1,700bp, that is, the complete coding sequence (999bp), the 5' (97bp) and 3' (565bp) untranslated regions. Several LDH genomic clones have also been isolated and partially characterized from mouse and human DNA libraries. Mouse genomic clone M15 appears to contain a functional gene of at least 8Kb consisting of seven exons and six introns. The relationship between the exon-intron organization of LDH-A gene and the structural-functional domains of LDH-A protein are illustrated. The nucleotide sequence of a human LDH-A pseudogene has been determined. 12.9% nucleotide differences were found between the pseudogene and LDH-A cDNA, and their significant implications on mammalian mutations and molecular evolution are observed. It is of interest to elucidate the genetic mechanism(s) underlying the tissue-specific expression of three LDH genes in human and other vertebrates.