To determine the expression and distribution of tegument proteins encoded by duck enteritis virus (DEV) UL51 gene in tissues of experimentally infected ducks, for the first time, an immunoperoxidase staining method to detect UL51 protein (UL51p) in paraffin-embedded tissues is reported. A rabbit anti-UL51 polyclonal serum, raised against a recombinant 6-His-UL51 fusion protein expressed in Escherichia coli, was prepared, purified, and used as primary antibodies. Fifty-eight 30-day-old DEV-free ducks were intramuscularly inoculated with the pathogenic DEV CHv strain as infection group, and two ducks were selected as preinfection group. The tissues were collected at sequential time points between 2 and 480 hr postinoculation (PI) and prepared for immunoperoxidase staining. DEV UL51p was first found in the spleen and liver at 8 hr PI; in the bursa of Fabricius and thymus at 12 hr PI; in the Harders glands, esophagus, small intestine (including the duodenum, jejunum, and ileum), and large intestine (including the caecum and rectum) at 24 hr PI; in the glandularis ventriculus at 48 hr PI; and in the pancreas, cerebrum, kidney, lung, and myocardium at 72 hr PI. Throughout the infection process, the UL51p was not seen in the muscle. Furthermore, the intensity of positive staining of DEV UL51p antigen in various tissues increased sharply from 8 to 96 hr PI, peaked during 120-144 hr PI, and then decreased steadily from 216 to 480 hr PI, suggesting that the expressional levels of DEV UL51p in systemic organs have a close correlation with the progression of duck virus enteritis (DVE) disease. A number of DEV UL51p was distributed in the bursa of Fabricius, thymus, spleen, liver, esophagus, small intestine, and large intestine of DEV-infected ducks, whereas less DEV UL51p was distributed in the Harders glands, glandularis ventriculus, cerebrum, kidney, lung, pancreas, and myocardium of DEV-infected ducks. Moreover, DEV UL51p can be expressed in the cytoplasm of various types of cells, especially most abundantly in the cytoplasm of lymphocytes, reticulum cells, macrophages, epithelial cells, and hepatocytes. The present study may be useful not only for describing the characteristics of UL51p expression and distribution in vivo but also for a greater understanding of the pathogenesis of this DVE.