A heat-labile inhibitor (HLI) capable of neutralizing the infectivity of variola virus, as measured by reduction of focus-forming and plaque-forming activity, was detected in normal human, guinea pig, rabbit and monkey sera. HLI lost its activity on incubation at 56° for 30 min, and showed different behaviors from complement in heat inactivation curves as well as in specific absorption with variola virions. HLI is suggested to be of globulin nature on the basis of sensitivity to trypsin digestion and reaction with fluorescent antibody against guinea pig serum γ-globulin, as determined by staining HLI-saturated virions. HLI was equally active for variola (major and minor), vaccinia and Newcastle disease viruses, but inactive for herpes simplex and Japanese encephalitis viruse