Bornside, George H. (Louisiana State University School of Medicine, New Orleans), Christopher B. Merritt, and Alice C. Weil. Reversal by ferric iron of serum inhibition of respiration and growth of Bacillus subtilis. J. Bacteriol. 87:1443–1451. 1964.—The inhibitory effect of serum and serum fractions on oxygen consumption by Bacillus subtilis was assayed manometrically; the inhibitory effect on growth was measured by a tube dilution assay, pour plates, and turbidimetrically. Although rabbit serum possessed high antirespiratory and bactericidal activity, a two-fold dilution of serum eliminated antirespiratory activity, but did not affect bactericidal activity. The bactericidal effect of rabbit serum was detected immediately upon mixture of serum and bacilli, but the antirespiratory effect was usually discernible after 2 hr. By these methods of assay, there was no statistically significant association between bactericidal and antirespiratory activities. This lack of close association between antirespiratory and bactericidal activities of serum was attributed to basic differences in the assay procedures, because bacterial growth during the manometric procedure itself was proportional to oxygen consumption. In these latter experiments, therefore, serum antirespiratory activity was proportional to serum bactericidal activity. Alcoholic fractionation of serum indicated that the antirespiratory agent was a globulin, but study of globulins from man and animals failed to identify any single globulin consistently as the antirespiratory agent. However, a highly purified preparation of human 7S γ-globulin exhibited the greatest specific antirespiratory activity, as well as completely inhibiting growth and respiration. Antirespiratory activity was slightly diminished after rabbit serum was heated at 56 C for 30 min, and appeared to be related to levels of unsaturated transferrin. Reversal of antirespiratory activity was obtained by addition of iron in excess of the amount needed to saturate transferrin; no reversal was obtained with aluminum, magnesium, manganese, copper, cobalt, or zinc. The effect of iron was on the serum and not on the bacilli. Oxygen consumption by B. subtilis was actually stimulated by highly purified rabbit transferrin. The antirespiratory agent was a serum globulin which was heat-stable and inactivated by iron, but was not transferrin.