CD44 is a widely expressed cell surface glycoprotein which is involved in both cell-matrix and cell-cell interactions which regulate a variety of processes, including leucocyte migration and activation. Therefore, we examined the expression of CD44, and its major ligand hyaluronan, during the induction and progression of experimental glomerulonephritis. Antibody staining of normal rat kidney showed constitutive CD44 expression by resident glomerular macrophages, parietal epithelial cells, medullary and occasional cortical tubules. There was a marked increase in CD44 expression over days 1, 7 and 21 of rat crescentic anti-glomerular basement membrane (GBM) glomerulonephritis. Infiltrating monocytes and lymphocytes were CD44+, with ultrastructural studies showing high levels of CD44 expressed on the surface of lymphocytes adherent to activated endothelium. Marked hyaluronan deposition was seen in areas of fibrosis on days 7 and 21, such as glomerular crescents and the periglomerular area. Hyaluronan deposition was accompanied by the presence of many CD44+ cells. Double immunohistochemistry showed that both CD44+ED1+ macrophages and CD44+ myofibroblasts (identified by expression of alpha-smooth muscle actin) were present in areas of fibrosis. There was also a dramatic increase in cortical tubular CD44 expression, which was most evident in areas of tubular damage. Although tubular epithelial cells expressed CD44 upon both the basolateral and luminal surface, CD44 expression was most prominent within tightjunctions, suggesting a role for CD44-CD44 interactions in cell-cell adhesion within the tubule. Analysis of CD44 isoforms by reverse transcriptase-polymerase chain reaction (RT-PCR) showed that the standard form of CD44 predominated in both normal and diseased kidney. However, a series of alternatively spliced CD44 isoforms was also detected, whose expression was markedly increased during disease. At least seven isoforms containing the v6 domain were identified, with the smallest form representing activated T cells. In conclusion, CD44 is constitutively expressed in normal kidney and is dramatically up-regulated in rat anti-GBM disease, suggesting possible roles for the CD44-hyaluronan interaction in leucocyte recruitment, renal fibrosis and tubular cell-matrix and cell-cell interactions during the induction and progression of crescentic glomerulonephritis.