Two non-lymphoid cell types play an important role in the pathogenesis of arthritis, i.e. the 'classical' macrophage and the antigen-presenting dendritic cell. In the present study, the roles of both cell types are studied in antigen-induced arthritis of the rat knee joint. Cryostat sections of whole, unfixed, undecalcified knee joints were used for immunohistochemical staining of non-lymphoid cells and lymphocyte subsets. For the demonstration of the different types of non-lymphoid cells, monoclonal antibodies against rat macrophages (ED1, ED2, and ED3) and against Ia antigen were used with an immunoperoxidase method. The results show in an overall view of the arthritic joint the different sites of action of the classical macrophages on the one hand and the Ia-positive dendritic cells on the other. Classical macrophages were mainly found in the superficial layers of the synovium bordering joint space and articular cartilage. Dendritic cells and T cells of the helper phenotype were mainly found in clusters surrounding small blood vessels within the synovium. These clusters express the immunological background of the antigen-induced arthritis and may well be responsible for the continuation of the arthritic process.