When rabbits are infected with Pasteurella multocida, the concentration of iron in their plasma decreases and their rectal temperature rises. To determine whether the rise in body temperature (fever) and the fall in plasma iron may be a coordinated host defense response, Pasteurella multocida were grown in vitro at various temperatures and iron concentrations. At afebrile temperatures the bacteria grew equally well at low or high concentrations of iron. However, when the temperature of the bath was raised to a febrile temperature the growth of the bacteria was inhibited by the low, but not the high, iron concentrations. These data support the hypothesis that one of the mechanisms behind the adaptive (or beneficial) role of fever is the reduced ability of pathogenic bacteria to grow well at elevated temperatures in an iron-poor medium.