The cell wall polymer lignin is believed to be condensed by specific cell wall-localized oxidoreductases. In many plants species, including poplar, the peroxidase-directed oxidation of the lignin analogue syringaldazine (SYR) has been localized to cells that undergo secondary wall formation, a process that includes lignification. As a first step to analyse the corresponding peroxidases, we have isolated previously two anionic isoenzymes (PXP 3-4 and PXP 5) from poplar xylem (Populus trichocarpa), which use SYR as a substrate. Here, we demonstrate that these enzymes are responsible for the visualized SYR oxidation in the developing xylem. The cDNA that corresponds to PXP 3-4 was isolated and the deduced protein was found closely related to the other SYR-oxidizing peroxidase PXP 5 (ca. 98% of identity). PXP 3-4 was expressed in a baculovirus expression system yielding high levels of active peroxidase (3 mg/l medium). The heterologously produced protein showed characteristics similar to those of the corresponding protein from poplar xylem (enzymatic properties, isoelectric point, and migration in a native gel). PXP 3-4 was expressed in the stem and in the root xylem. The data demonstrate that PXP 3-4 (and/or PXP 5) are present in differentiating xylem, supporting a function in secondary cell wall formation.