The amino acid sequence of the gene for the peplomer protein of the vaccine strain M41 and the Beaudette laboratory strain M42-Salk of avian infectious bronchitis virus (IBV) have been derived from cDNA sequences. As found with other coronaviruses, the peplomer protein carries the epitopes eliciting neutralizing antibodies. The gene encodes a primary translation product of 1162 amino acids with a molecular weight of 128079. The use of a recent algorithm to predict membrane-protein interactions led to the unambiguous localization of the signah peptide and a transmembrane anchor ฮฑ-helix at the C-terminus. At 50 positions amino acid differences were found between M41 and two Beaudette strains (M42-Salk and M42-Houghton). They are partly clustered in two regions of the protein. These two regions are candidates for neutralization epitopes of the protein. ยฉ 1986.