question: The cleavage products of the spike (S) protein, the S1 and S2 subunits, of the highly neurovirulent murine coronavirus (MHV) JHMV c1-2 variant were identified by immunoprecipitation of virus-infected cell lysates after treatment with urea and 2-mercaptoethanol. By this method 14 monoclonal antibodies (MAbs) raised against the S protein of the c1-2 variant were revealed to react with the S1 subunit and one with the S2 subunit. These 14 MAbs were classified into the following three groups: (A) MAbs reactive to almost all MHV strains examined, (B) MAbs specific for the JHMV strain and (C) MAbs specific for a large S protein of the JHMV strain. All five MAbs classified in group B showed neutralization activity and four of them also showed fusion inhibition activity. Four of six MAbs in group C showed neutralizing activity to the c1-2 variant but not to the sp-4 variant, and most of them had no fusion inhibition activity. Western blot analyses showed that all of the MAbs, except for no. 2 in group A, failed to react with the denatured S and S1 proteins. All MAbs in groups A and C, with the exception of no. 19 in group A, reacted with the mildly denatured S proteins, whereas none of the MAbs in group B did. These results suggest that MAbs in group B recognized highly conformational epitopes which may be involved in the binding of virions to cellular receptors and the fusion activity of the virus.