Several serological interrelationships between various members of the corona-virus group have been revealed in neutralization, complement fixation, and gel-diffusion tests, using human and hyperimmune animal sera. Several members of this group of human and animal pathogens are shown to cross-react in one or more type of test, but one member, avian infectious bronchitis virus, was shown to be unrelated. Mouse hepatitis virus (MHV3) was found to be antigenically related to a number of human types of coronavirus. Difficulties were encountered in the investigation of paired human sera in demonstrating the specificity of antibody rises, placing doubt on the values of some serological studies. The significance of these interrelationships is discussed in the light of other investigations.