Decay-accelerating factor (DAF or CD55) and CD59 are regulators that protect self cells from C3b deposition and C5b-9 assembly on their surfaces. Their relative roles in protecting glomeruli in immune-mediated renal diseases in vivo are unknown. We induced nephrotoxic serum (NTS) nephritis in Daf1−/−, CD59a−/−, Daf1−/− CD59a−/−, and wild-type (WT) mice by administering NTS IgG. After 18 h, we assessed proteinuria, and performed histological, immunohistochemical, and electron microscopic analyses of kidneys. Twenty-four mice in each group were studied. Baseline albuminuria in the Daf1−/−, CD59a−/−, and Daf1−/− CD59a−/− mice was 82, 83, and 139 as compared with 92 μg/mg creatinine in the WT controls (p> 0.1). After NTS, albuminuria in CD59a−/− and WT mice (186±154 and 183±137 μg/mg creatinine, p> 0.1) was similar. In contrast, Daf1−/− mice developed severe albuminuria (378±520, p< 0.05) that was further exacerbated in Daf1−/− CD59a−/− mice (577±785 μg/mg creatinine, p< 0.05). Glomerular histology showed essentially no infiltrating leukocytes in any group. In contrast, electron microscopy revealed prominent podocyte foot process effacement in Daf1−/− mice with more widespread and severe damage in the double knockouts compared with only mild focal changes in CD59a−/− or WT mice. In all animals, deposition of administered (sheep) NTS Ig was equivalent. This contrasted with marked deposition of both C3 and C9 in Daf1−/− CD59a−/− and Daf1−/− mice, which was evident as early as 2 h post-NTS injection. The results support the proposition that in autoantibody-mediated nephritis, DAF serves as the primary barrier to classical pathway-mediated injury, while CD59 limits consequent C5b-9-mediated cell damage.