A 153 bp fragment of the cDNA encoding the beta-subunit of pig heart NAD(+)-isocitrate dehydrogenase (NAD(+)-ICDH) was specifically amplified by PCR, using redundant oligonucleotide primers based on partial peptide sequence data [Huang and Colman (1990) Biochemistry 29, 8266-8273]. This PCR fragment was then used as a probe to isolate cDNA clones encoding the complete mature form of the beta-subunit from a monkey testis cDNA library. Examination of the deduced amino acid sequence of the monkey subunit and the partial sequence of the pig heart enzyme revealed a high level of sequence conservation. In addition, 3 overlapping fragments of the cDNA for the alpha-subunit of monkey NAD(+)-ICDH were amplified using oligonucleotide primers derived from the cDNA sequence of a subunit of bovine NAD(+)-ICDH (EMBL accession no: U07980). These cDNA fragments allow deduction of the amino acid sequence of the alpha-subunit. Since the gamma-subunit of monkey NAD(+)-ICDH has already been cloned [Nichols, Hall, Perry and Denton (1993) Biochem. J. 295, 347-350], a deduced amino acid sequence is now available for all three subunits of mammalian NAD(+)-ICDH. Interrelationships between these subunits are discussed and they are compared with the two subunits of yeast NAD(+)-ICDH and Escherichia coli NADP(+)-ICDH.