A full length cDNA for human M creatine kinase has been isolated and sequenced. The cDNA contains 77 bp of 5' untranslated, 338 bp of 3' untranslated sequence and the entire coding region (1146 bp) for human M creatine kinase. The M creatine kinases from different species share considerable sequence homology within the coding region (77-91%) and in amino acid sequence (82-97%). Little or no sequence homology is observed in the 3' untranslated sequence of the mammalian M creatine kinases, although canine and human creatine kinase share overall 80% sequence homology in 5' untranslated sequence. A unique 8 bp sequence was identified in the 5' untranslated regions of mammalian M creatine kinase but is not present in B creatine kinase cDNA. The degree of sequence conservation observed implies an evolutionary constraint on M creatine kinase structure beyond that which would be expected for the maintenance of enzymatic function.