Pyogranulomas of ovine caseous lymphadenitis (CLA) are encapsulated lesions resulting from infections with Corynebacterium pseudotuberculosis, a bacterial pathogen able to grow within macrophages. Immunohistology of CLA lesions showed a band of lymphocytes lining the inside of the collagen capsule in intimate contact with necrotic tissue, the intracapsular lymphocytes being organized into three layers. The innermost layer, immediately adjacent to the central necrotic tissue consisted of a narrow band of MHC class II staining macrophages. Cells staining for CD4, CD8 and gamma delta T cell markers were unevenly distributed throughout the lymphoid layer, tending to be more numerous immediately external to the macrophage layer. The intracapsular lymphoid tissue contained a high proportion of CD8+ lymphocytes (CD4:CD8, 1.5:1) and of gamma delta lymphocytes (CD4:CD8:gamma delta, 1:0.7:0.8). External to the T cell-rich zone and adjacent to the surrounding collagen capsule was a dense band of cells, a proportion of which stained atypically for CD45R and were tentatively identified as B cells. CD8+ and gamma delta+ T cells showed similar distributions and their relative abundance, compared with CD4+ T cells, was a distinguishing feature of the CLA lesion. Staining for factor VIII-related antigen clearly showed endothelial venules throughout the intracapsular lymphoid tissue. The presence of endothelial venules and the organized architecture of the lymphoid tissue teleologically argues that lymphocytes are continually recruited into chronic CLA lesions and play an important role in the ongoing disease process.