We have isolated, cloned and characterized a cDNA from Zea mays L., denoted ZmAP1, coding for an anionic peroxidase. The open reading frame of ZmAP1 starting 72 residues from the 5' end of the cDNA predicts a 37,778 dalton protein of 356 amino acid residues. The protein has high similarity to other peroxidases and contains two peroxidase motifs that carry two highly conserved histidines in the active center. We expressed recombinant ZmAP1 protein in E. coli as a fusion with maltose-binding protein. The fusion protein was biochemically active after addition of hemin to the apoprotein. The maize peroxidase ZmAP1 has a pH optimum at pH 4.0 and a Km of 0.2 mM for the substrate 2,2'-azino-bis-(3-ethyl-benzothiazolin-6-sulfonic acid) at this pH. In maize seedlings the ZmAP1 gene is expressed predominantly in roots, the mesocotyl, the coleoptile and to a lower extent in the node, whereas no expression in the primary leaf was found. In situ hybridization shows that the expression of ZmAP1 in the young maize root is confined to the epidermis, hypodermis and the pericycle.