Hybrid cells secreting monoclonal antibodies against Bovine Enteric Coronavirus (BECV strain G110) were obtained by fusion between SP2/O myeloma cells and splenocytes of mice hyperimmunized with purified virus. Specificity of 8 from the 12 monoclonal antibodies was established to be anti-GP105 by immunochemical staining of viral polypeptides and by immunoprecipitation of radioactive-labelled viral proteins. The reactivity of three BECV isolates (strain G110, F15 and NCDCV) with the different monoclonal antibodies was studied by indirect immunofluorescence (IIF), ELISA, hemagglutination-inhibition and seroneutralization. G110 and F15 interacted in a similar manner with the monoclonal antibodies, but NCDCV failed to react with A9 monoclonal antibody. IIF test allowed us to detect another difference between U.K. BECV isolate and other BECV, in that the U.K. isolate did not react with F7 monoclonal antibody. No differences could be seen by IIF in the reactivity of Danish BECV isolate, Human Enteric Coronavirus (HECV), Bovine Respiratory Coronavirus (BRCV) and our BECV strains, with the 12 monoclonal antibodies. Human Respiratory Coronavirus (OC43), Porcine Hemagglutinating Encephalitis Virus (HEV) and BECV share common antigenic determinants which are outlined by the 6 non-neutralizing monoclonal antibodies. On the basis of the reactivity of coronaviruses with anti-BECV monoclonal antibodies, a distinction can be easily made between BECV and other coronaviruses, and differences were also detected within the BECV isolates. Interestingly HECV were found to be more closely related to BECV than to human OC43. These results show that monoclonal antibodies are of valuable help for biochemical and antigenic studies as well as for diagnostic procedures.