Intravenous injection of ferric ammonium citrate equivalent to 5 mg iron/kg abolished the protective effect of Clostridium welchii antiserum in guinea-pigs infected with Cl. welchii type A. In animals not given iron there was an abrupt cessation of growth 4–5 hours after infection. In those given iron bacterial growth was continuous and the animals died in 12–24 hours. In both of the animals there was always an excess of Cl. welchii alpha antitoxin in the plasma and tissues. Injection of iron did not interfere with phagocytosis of Cl. welchii. Iron abolished the protective effect of antiserum up to 6 hours after infection. It had no effect when given 8 hours after infection or later.