An indirect immunoperoxidase method was used to localize the Ia, Thy-1 and MRC OX 2 antigens in rat lymphoid tissues. The antigens were detected using mouse monoclonal antibodies and all were notable since they were found to be expressed on different dendritic or reticular elements in lymphoid tissue. Ia antigen was present on bone marrow-derived dendritic cells in the T-dependent areas of spleen and lymph nodes in addition to B cells. MRC OX 2 antibody labelled dendritic cells in the follicles of spleen and lymph nodes but not the Ia positive cells in the T-dependent areas. Cells associated with blood vessels including the endothelium of the post capillary venules were also labelled with MRC OX 2 antibody. In contrast, anti-Thy-1 antibody labelled the pericyte sheath surrounding the post capillary venules and connective tissue elements in lymphoid tissues. The odd patterns of distribution of these antigens are discussed with respect to possible functions of the antigens and the cells they label.