Abstract: The present invention relates to a novel assay for lymphocyte chemotaxis. The assay is transendothelial assay using endothelial cells cultured on microporous filters. Lymphocyte transmigration through the filter toward a suspected chemoattractant is measured. Apparatuses for carrying out the assay are also provided. The apparatuses and methods of the present invention can be used for the identification of inhibitors (e.g., antagonists) or promoters (chemoattractants) of the adhesion receptor-mediated migration of leukocytes through the endothelium (extravasation). Such inhibitors and promoters respectively inhibit and promote the inflammatory response, and thus have therapeutic utilities. The inhibitors and promoters are identified by detecting their abilities to respectively inhibit or promote the chemotaxis of lymphocytes in the assay of the invention. The assay of the invention also has diagnostic utilities for detecting a disease or disorder involving a defect in lymphocyte chemotaxis.