Improvement of the mouse hepatitis virus (MHV) antigen in a prototype ELISA kit was performed. Equivalent divalent antigens of MHV Nu-67 and S strains with a protein concentration of 10 micrograms/ml showed the best sensitivity and specificity for the detection of MHV and sialodacryoadenitis/Parker's rat coronavirus antibodies in mice and rats, respectively. An increase in the reliability of macroscopic evaluation of both antibody tests is expected by using the newly manufactured kit with the improved antigen.