We have examined the direct interaction of lymphocytes with human fibroblasts in vitro. The results of our study, using radiolabelled lymphocytes, show that activated T cells rapidly adhered to the fibroblasts when the two types of cell were cultured together at 37 degrees, and that the residency time of lymphocyte-fibroblast contact was approximately 30 min. Lymphoblast adhesion did not require the presence of divalent cations, but it was temperature-dependent. Although the binding of the lymphocytes was not prevented by the presence of monosaccharides or by treatment with exoglycosidases, it was highly sensitive to low concentrations of trypsin. Attachment was also inhibited partially in the presence of the sulphated fucose polysaccharide, fucoidan. These results suggest that at least two distinct receptors are involved in lymphocyte adhesion to fibroblasts. The binding of B lymphoblasts to fibroblasts was much greater than that of the activated T cells. Resting, non-stimulated, B and T lymphocytes both bound only very poorly, showing that only mature lymphocytes express high levels of functionally effective adhesion proteins. Moreover, the results of competition experiments suggest that different cell surface components mediate the interaction of T and B cells with fibroblasts.