An indirect immunofluorescence test, using cultured cells infected with two strains of mouse hepatitis virus, was developed for detection of antibody to rodent coronaviruses. The immunofluorescence test detected serum antibody to mouse hepatitis virus and rat sialodacryoadenitis virus earlier than the neutralization test. In the case of mouse hepatitis virus, the results of the immunofluorescence test closely paralleled those obtained with a commercially available enzyme-linked immunosorbent assay.