CD59, a potent inhibitor of the human membrane attack complex (MAC) of complement, is present on many different tissues throughout the body. Recently we identified and characterized the rat analogue of CD59 and produced a number of monoclonal antibodies (mAb). We have now used these antibodies to demonstrate, by immunofluorescence microscopy, that rat CD59 was widely expressed in the rat kidney. Staining of renal sections revealed the presence of rat CD59 in abundance in the glomerulus, collecting ducts and distal tubules. Staining was abolished by treatment of sections with phosphatidylinositol-specific phospholipase C (PIPLC). Rat mesangial cells also stained strongly for rat CD59, giving an intensely granular staining pattern. In complement attack experiments the cultured mesangial cells were rendered more susceptible to lysis by rat or human complement after preincubation with the anti-inhibitor mAb. The results demonstrate that the rat analogue of CD59 is present and functionally active in rat renal tissue. The protective effect of CD59 against MAC-mediated tissue injury can now be examined in rat models of human renal disease.