This report describes the immunocytochemical detection and distribution of different canine leucocytes antigens (Thy-1, CD5, CD4, CD8, MHC-II and B-cell antigen) by means of monoclonal antibodies (mAbs) in frozen tissue sections of lymphoid organs of the dog. CD5 is expressed exclusively by T lymphocytes. Thy-1 is expressed by T-lymphocytes and in a variety of non-lymphoid tissues. The mAb against canine MHC-II could detect B and T lymphocytes, macrophages, fibroblasts and antigen presenting cells. The CD4 antigen is expressed on many lymphocytes of the T-dependent areas of lymph organs and is also expressed by certain antigen presenting cells. CD8 is expressed in a minor proportion of T cells. The mAb against B cells detected lymphocytes in lymphoid follicles and epithelial cells of the thymic medulla. © 1995.