Summary: Antigenic comparison of seven organ culture-grown human coronavirus strains, one tissue culture-grown human strain (229E), several strains of mouse hepatitis virus (MHV) and one strain of infectious bronchitis virus (IBV) was made. With the use of neutralization, complement fixation and fluorescent antibody tests, it was shown that strain IBV-42 was unrelated to the human and murine viruses and that strain 229E bore little, if any, relation to the murine or other human strains tested. In addition, three of the organ culture grown viruses, two of which have been adapted to growth in suckling mice, were shown to be antigenically related to several strains of MHV. The four remaining human strains possessed heterogeneous and poorly defined antigenic characteristics.