A cDNA clone for glutaminyl cyclase was isolated from a human pituitary cDNA library and the complete DNA sequence determined. The cDNA clone had 1573 bp and contained an open reading frame of 1086 bases, coding for a protein of 361 amino acids and molecular mass of 40,876 Da. The predicted amino acid sequence of the human cDNA showed 86% sequence identity to the previously reported bovine glutaminyl cyclase sequence. A comparison of the amino acid sequences derived from the human and bovine cDNAs showed that several glycosylation and phosphorylation sites as well as two cysteine residues (Cys139, Cys164) were conserved. The human cDNA was cloned into the Escherichia coli expression vectors pMALc2 and pET19b. Expression of this cDNA in either vector resulted in the production of a glutaminyl cyclase fusion protein which was enzymatically active and reacted with anti-bovine glutaminyl cyclase antisera. Substrate specificity studies with the recombinant enzyme suggested a bias against acidic and tryptophan residues adjacent to the N-terminal glutaminyl residue and a lack of importance of chain length after the second residue.