The membrane attack complex (MAC) of complement is activated by immune and non-immune mechanisms in the kidney. MAC has been found associated with glomerular immune deposits, but also to cell remnants, particularly along tubules and in vessel walls. Clusterin and S-protein (vitronectin) bind to MAC, rendering it cytolytically inactive. Both have been found associated with MAC in renal tissue. Here we analysed the deposition of clusterin and S-protein in 118 renal biopsies relative to the localization of the MAC using MoAbs. Statistical analysis was performed comparing no or little versus evident or strong staining by immunofluorescence (IF). In glomeruli, out of the 92 biopsies where both MAC and immunoglobulins were evaluated, deposits of MAC were found in the presence (32 out of 41) but also in the absence of immunoglobulins (20/51). Clusterin and S-protein deposits were seen, respectively, in 25 out of 61 and 36 out of 61 biopsies containing glomerular MAC, and almost never in its absence (one out of 50 for both). The association of the two inhibitors with MAC was observed mainly in glomeruli containing immunoglobulin deposits (respectively, 21 out of 32 and 25 out of 32), but not when immunoglobulins were absent (three out of 20 and seven out of 20) (coefficient of concordance, K = 0.47 and 0.43). The localization of MAC along tubules and in vessels was easily identified in most biopsies (93 out of 118) and was accompanied by S-protein in most cases (tubules, 86 out of 93; vessels, 82 out of 93) (K = 0.58 and 0.57 respectively) but not by clusterin (28 out of 93 and 24 out of 93). These results suggest that clusterin does not co-localize with MAC whenever there is formation and fixation of the MAC. It seems that clusterin has a particular affinity for MAC which is associated with immunoglobulin. This observation should help to distinguish between the different forms of MAC, and might indicate that MAC associated with immunoglobulin is essentially in its cytolytically inactive form.