The complete sequence of the spike (S) gene of the Br1/87 isolate of porcine epidemic diarrhoea virus (PEDV) was determined from cDNA clones. The predicted polypeptide was 1383 amino acids long, contained 29 potential N-linked glycosylation sites and showed structural features similar to those of the coronavirus spike protein. The PEDV S protein, like that of the members of the transmissible gastroenteritis virus (TGEV)-related subset, lacks a proteolytic site to yield cleaved amino and carboxy subunits S1 and S2. Viral polypeptide species of the expected M r, i.e. 170K/190K, were observed in PEDV-infected cells. Sequence comparison confirmed that, within the subset, PEDV was most closely related to the human respiratory coronavirus HCV 229E. However, PEDV S protein has an additional 250 residue N-terminal domain which is absent from HCV 229E and porcine respiratory coronavirus, the respiratory variant of TGEV. Alignment of the S1 regions revealed a second domain of about 90 residues with increased sequence divergence which might possibly express virus-specific determinants.