The membrane attack complex of complement (MAC) plays an important role in the mediation of proteinuria in experimental membranous nephropathy induced by Heymann antiserum. SP-40,40 is a recently described serum protein which appears to inhibit the formation of cytolytic MAC in a manner analogous to S protein/vitronectin. SP-40,40 is homologous to proteins originally isolated from rat and ram seminal fluid (sulfated glycoprotein 2 and clusterin, respectively). By current convention, these proteins are considered clusterin homologues. The objective of this study was to examine the participation of rat clusterin in passive Heymann nephritis. Using an antibody to rat clusterin as an immunofluorescent probe, clusterin deposits were demonstrated along the glomerular capillary wall in an identical pattern to rat C3 and C5b-9. Decomplementation using cobra venom factor prevented proteinuria and intraglomerular MAC formation. The epimembranous clusterin were not detected in the complement-depleted animals. The role of clusterin in the mediation of glomerular injury remains unknown, but it is probably related to in situ formation of the terminal complement cascade where it may play a regulatory role.