The cloning, expression and characterization of plant agmatine iminohydrolase (AIH, also known as agmatine deiminase, EC 3.5.3.12) is described. Recombinant AIH of Arabidopsis thaliana forms dimers and catalyzes the specific conversion of agmatine to N-carbamoylputrescine and ammonia. Biochemical data suggested that cysteine side chains are involved in catalysis. However, site-directed mutagenesis of the two highly conserved cysteine residues of AIH showed that these cysteines are important but not essential for activity, arguing against a thioester substrate-enzyme intermediate during catalysis. This work represents the completion of the cloning of the arginine decarboxylase pathway genes of higher plants.