This report describes the identification of a novel linear B-cell epitope at the C-terminus of the membrane (M) protein of avian infectious bronchitis virus (IBV). A monoclonal antibody (MAb) (designated as 15E2) against the IBV M protein was prepared and a series of 14 partially-overlapping fragments of the IBV M gene were expressed with a GST tag. These peptides were subjected to enzyme-linked immunosorbent as say (ELISA) and western blotting analysis using MAb 15E2 to identify the epitope. A linear motif, $^{199}FATFVYAK^{206}$, which was located at the C-terminus of the M protein, was identified by MAb 15E2. ELISA and western blotting also showed that this epitope could be recognized by IBV-positive serum from chicken. Given that 15E2 showed reactivity with the $^{199}FATFVYAK^{206}$ motif, expressed as a GST fusion protein, in both western blotting and in an ELISA, we proposed that this motif represented a linear B-cell epitope of the M protein. The $^{199}FATFVYAK^{206}$ motif was the minimal requirement for reactivity as demonstrated by analysis of the reactivity of 15E2 with several truncated peptides that were derived from the motif. Alignment and comparison of the 15E2-defined epitope sequence with the sequences of other coronaviruses indicated that the epitope is well conserved among chicken and turkey coronaviruses. The identified epitope should be useful in clinical applications and as a tool for the further study of the structure and function of the M protein of IBV.