SUMMARYBoth the haemagglutinin and the complement fixing antigen of ectromelia virus are fairly stable at hydrogen ion concentrations ranging from pH 4 or 5 to 11, although the highest titres were found around the neutral point.Ectromelia complement fixing antigen showed some drop in titre on heating at 56°C. for 30 minutes, but no more so than vaccinial preparations run in parallel.The haemagglutinin seems to have a heat labile as well as a heat stable component. The heat labile component was neutralised by rabbit anti‐serum but not by normal rabbit serum and so appears to be specific.