The staining pattern of a group of adhesion molecules on the immunoreactive cells in the lining layer of lymphocytic infiltrates of the rheumatoid synovial membrane was studied, using monoclonal antibodies by immunoperoxidase staining method against LFA-1, VLA-4, VLA-5, ELAM-1, and ICAM-1. The cells of the lining layer were strongly ICAM-1+ and VLA-5+, suggesting (1) that ICAM-1 may function to facilitate the adhesion of ICAM-1 bearing type A cells to type B cells, and (2) that the lining cells may utilize VLA-5 for anchorage to fibronectin at the surface of the synovial membrane. In the lymphocyte-rich and transitional area, the endothelial cells of the postcapillary venules were both ELAM-1+ and ICAM-1+. ICAM-1 staining of mononuclear cells was weak in lymphoid aggregates, but strong in the transitional area, indicating a paucity of ICAM-1 bearing cells in the lymphocyte-rich areas. On the other hand, LFA-1 staining was very strong in the lymphoid aggregates and only moderate in transitional areas. This suggests that the large numbers of T4 cells in the lymphocyte rich areas are sufficiently activated to express substantial levels of LFA-1, and also that the LFA-1 molecule is an important receptor for emigration from postcapillary venules. In germinal center-like areas in lymphoid aggregates, most of the cells stained strongly for ICAM-1 and VLA-4, suggesting that the proliferation of B lymphocytes may be facilitated by LFA-1 and VLA-4 dependent T and B cell interaction. The VLA molecules stained in the transitional areas may provide appropriate adhesion and anchorage for the achievement of the variety of immune reactions which occur in these areas.