The interaction between lymphocytes and fibroblasts in vitro has been examined using a quantitative ELISA assay to measure the binding of T and B cells to monolayer cultures of human dermal fibroblasts. This was carried out on microtiter culture plates, using an anti-Thy-1 monoclonal antibody, to determine the attachment of murine T lymphocytes and an affinity-purified polyclonal anti-IgM antibody to measure B cell binding. Both types of lymphocyte were found to adhere strongly to intact human fibroblasts, and also had high levels of attachment to purified fibroblast plasma membranes and extracts of the fibroblast extracellular matrix. Attachment, particularly of B lymphocytes, also took place onto plastic surfaces coated with fibronectin, but not to collagens or to intact fibroblasts that had been fixed with a low concentration of paraformaldehyde. Lymphocyte binding to fibroblasts was partially prevented by a monoclonal antibody against fibroblast MHC class II antigens, but not against the class I membrane complex, or by polyclonal antiserum to the cell surface mannose 6-phosphate receptor. In addition, although both lymphocyte types were able to adhere to fibro-nectin, the presence of antibody against fibronectin or the synthetic peptide Arg-Gly-Asp-Ser, had no effect on their attachment to fibroblasts. Thus, lymphocyte adhesion may occur by fibronectin, but other types of interactions with fibroblasts also appear to take place.