IT HAS been known for some time that the neutralizing ability of antiserum to certain viruses is considerably affected by whether or not the serum has been heat inactivated.1-3 Rubella belongs within this group, and there are numerous reports on the enhanced activity of fresh serum as compared to heat inactivated4,5 and also on the enhancing effect of the addition of fresh control serum to heated antiserum.5-7 Another virus sensitive to the heat labile components of antiserum is avian infectious bronchitis virus; recent electron microscope studies have shown that a type of lysis similar to hemolysis can be visualized by the negative staining procedure when unheated antiserum is combined with the virus.8,9 This lysis appears as distinct pits or craters approximately 100 Angstroms in diameter in the lipoprotein membrane of the virus (Fig 1, A and B). Having previously shown that rubella could be visualized as