Immunohistochemistry was used to characterize the distribution of leucocyte subsets in the bronchial mucosa of 11 dogs with idiopathic eosinophilic bronchopneumopathy (EBP). Formalin-fixed tissues from all dogs were included in the study, but frozen tissue from only one dog was available. MHC class II + cells were found in moderate numbers in the lamina propria (LP). These cells were morphologically either dendritic-like cells or macrophages, but many macrophages did not express MHC class II. Such molecules were expressed by occasional fibroblasts. L1 + cells, which formed a relatively small component of the LP inflammatory infiltrate, were morphologically either macrophages or polymorphonuclear cells (probably eosinophils). IgA + plasma cells were found in varying numbers in the LP, mostly in association with glandular tissue. IgG + plasma cells were less common, and IgM + plasma cells were present in low numbers. Many CD3 + cells were present in the LP. In the single case from which frozen tissue was available, most of the lymphocytes were labelled with CD4 marker, while smaller numbers were CD8 + T cells. Most of the lymphocytes in this case were positively labelled with T-cell receptor (TCR)-αβ marker. TCR-γδ + cells, although less common, were present in significant numbers throughout the LP. CDlc + dendritic cells were numerous in the epithelium and in the LP, immediately beneath the basement membrane. These findings, which were similar to those described in human asthma, are suggestive of a Th2 dominant immune response in canine EBP. As in human asthma, this provides a possible basis for new forms of treatment in canine EBP.