The appearance of different macrophage subpopulations, Ia-positive antigen-presenting dendritic cells and of T and B lymphocytes was studied in early phases of antigen-induced arthritis in rat knee joints. Cryostat sections of whole knee joints were analysed with immunohistochemical techniques using monoclonal antibodies against rat macrophages, Ia-antigen, and lymphocyte subpopulations. The results showed that in the early phases of the development of arthritis, the synovium was already infiltrated by many monocytes, young macrophages, granulocytes, perivascular Ia-positive non-lymphoid cells, some mature tissue macrophages, and only few T lymphocytes. In later phases not only monocytes, young macrophages and Ia-positive cells became more prominent but also the more mature ED2 positive macrophages and the ED3 positive macrophages that are normally confined to lymphoid organs became increasingly important. The T-cell population increased to some extent in later phases of arthritis induction, possibly induced by clustering with the Ia-positive cells.