Viral proteins of two strains of infectious bronchitis virus (IBV), which have 
different tissue trophism and serology, were separated on the basis of their 
isoelectric points (pI). The viruses have four structural proteins; the protein 
of greatest serological importance is found at the peplomer tip. The viral 
structural proteins separated by isoelectric focusing were identified by 
comparison to SDS-PAGE separations. Three protein bands were identical in pI and 
one protein band showed a difference in pI between strains. When the renatured 
viral proteins were Western blotted and reacted with strain-specific antiserum, 
antigen-antibody complexing was seen only at points corresponding to the 
strain-specific variant bands. For IBV strain Mass-41, antigen-antibody 
complexing occurred at a pI of 6.8, and, for IBV strain Ark-99, at 7.2. No cross 
reaction of antisera was observed for either strain. Since tissue affinities are 
a function of the viral peplomer-mediated attachment of virus to cells and are 
often directly related to pathogenicity, it appears that altered pathogenicity 
of strains of IBV may be detected by alteration of pI of the proteins. 
Classification by pI of proteins of at least the smaller viruses allows 
untypeable, highly pathogenic or persistent strains of these viruses to be 
characterized on the basis of variant proteins.