Human torovirus (HTV) and Breda virus (BRV), members of the genus torovirus in the family Coronaviridae, are established infectious agents of humans and cattle, respectively. The hemagglutinin-esterase (HE) gene of Breda virus serotype 2 (BRV-2) has been identified and the nucleotide sequence for BRV serotype 1 (BRV-1) genome which contains the open reading frames for the viral structural proteins has been reported revealing the presence of a 1. 25 kb gene whose nucleotide sequence is identical to that of the BRV-2 HE gene. In this study, we amplified the 1.2kb HE gene from the HTV genome using long RT-PCR and sequenced the amplicon directly. At the nucleotide level, the HTV HE gene manifests 85% sequence identity to the HE genes of BRV-1 and BRV-2 and 89% identity with the X pseudogene sequence of BEV. The 1.25 kb amplicons which contained the HE genes of BRV-1 and HTV were cloned and expressed in a baculovirus system and the proteins purified by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Hyperimmune sera prepared in guinea pigs against these proteins were reactive with both bovine torovirus (BTV) and human torovirus (HTV) antigens. By immunoblot, they reacted specifically with a 65 kDa protein corresponding in size to the torovirus HE protein. Furthermore, the hyperimmune sera but not the preimmune sera reacted with a series of BTV-positive and HTV-positive fecal specimens by immunoblot and dot blot analysis. By immunoelectron microscopy (IEM) torovirus particles from BTV-positive specimens from calves with diarrhea and HTV-positive specimens from patients were aggregated by the hyperimmune sera. Human convalescent sera and gnotobiotic calf post-infection sera reacted by immunoblot with the expressed 65 kDa protein. The expressed HE protein of HTV has important diagnostic potential.