The effect of inoculation with Escherichia coli on serum iron concentrations of turkeys and the effect of exogenous iron as ferric ammonium citrate, on E. coli septicemia in turkeys were determined. Inoculation of air sacs with E. coli produced hypoferremia in 18-day-old turkeys. Administration of iron with E. coli significantly increased mortality, frequency and degree of bacteremia, and severity of lesions in inoculated turkeys, compared with those turkeys given E. coli but not given iron. Similar results were seen whether iron was inoculated at the same location as E. coli or at a different location;Rabbit antiserum was prepared against iron-regulated outer membrane proteins of E. coli. Eighteen-day-old turkeys were passively immunized with rabbit antiserum and challenged by air sac inoculation of 1 to 2 x 10('6) colony-forming units of E. coli (078:K80:H9). Turkeys injected with normal rabbit serum or saline solution prior to challenge served as controls. Fatalities (8 of 51 inoculated) occurred only in groups given saline solution or normal rabbit serum. Remaining turkeys were necropsied 96 hours after challenge. Passive immunization with antiserum significantly reduced the frequency of bacteremia at 96 hours after challenge, the frequency of recovery of E. coli from air sacs, and the severity of gross lesions in inoculated birds as compared to birds given normal rabbit serum or saline solution before challenge.