We examined the expressions of alpha-subunits of very late antigen (VLA)-4 (alpha4) and VLA-5 (alpha5) on tonsillar lymphocytes and the interaction between these integrin receptors and their ligand, fibronectin (FN). Immunohistological and flow cytometric analyses showed that alpha4 and alpha5 were expressed in the lymphoid follicle and were positive on about 10% each of T cells and on 55% and 35% of B cells. When tonsillar B cells were separated by a discontinuous Percoll gradient, the number of alpha4- and 5-positive cells decreased as the cell density went down, while the number of activated cells went up. After in vitro activation of tonsillar B cells by Staphylococcus aureus Cowan I strain (SAC), the expression of alpha5 and the adhesiveness to FN decreased. The increased proliferation of B cells was observed when tonsillar B cells were cultured with immobilized FN. The expressions of alpha-subunits of VLA-4 and VLA-5 on tonsillar T and B lymphocytes increased with age. These results suggest that: i) B cell activation may cause decreased expressions of VLA-4 and -5, which gives a costimulatory effect on B cell activation itself again in cooperation with FN, ii) Increased expressions of VLA-4 and -5 on tonsillar lymphocytes with age may be related to regional immune response of the palatine tonsils.