Growth-arrest specific gene 6 (Gas6) is a vitamin K–dependent growth factor for mesangial and epithelial cells. To investigate whether Gas6 is essential for progressive glomerular injury, we constructed Gas6–/–mice and examined the role of Gas6 in accelerated nephrotoxic nephritis (NTN), a model of progressive glomerulonephritis. We found less mortality and proteinuria in Gas6–/–mice than in wild-type mice following injection of nephrotoxic serum. Glomerular cell proliferation, glomerular sclerosis, crescent formation, and deposition of fibrin/fibrinogen in glomeruli were also reduced in Gas6–/–mice. Furthermore, administering Gas6–/–mice recombinant wild-type Gas6, but not Gas6 lacking a previously characterized N-terminal γ-carboxyl group, induced massive proteinuria, glomerular cell proliferation, and glomerulosclerosis, comparable to responses seen in wild-type mice. These data indicate that Gas6 induces glomerular cell proliferation in NTN and suggest that this factor contributes to glomerular injury and the progression of chronic nephritis.