Coronaviridae are enveloped RNA viruses which mature by budding into intracytoplasmic membranes. Coronaviruses cause respiratory and/or enteric infection in humans and many domestic animals (reviewed by Wegeet al., 1982). The prototype Coronavirus is avian infectious bronchitis virus (IBV, Tyrrell et al., 1978). Coronaviridae exhibit rather fastidious requirements for the species and tissue types which they will infect. Because of the difficulty of isolating coronaviruses, most of them were first classified by their virion morphology in negatively stained preparations (Tyrrell et al., 1975). The virions are characterized by large, club-shaped peplomers or spikes about 20 nm long and 7 nm wide at their tips (Almeida and Tyrrell, 1967; Tyrrell et al., 1968; Fig. 7-1). Avian coronaviruses vary in diameter from 70 to 120 nm, but murine coronaviruses have more uniform diameters, approximately 90 nm. More recently, common features of virus replication and biochemistry have confirmed the assignment of coronaviruses into a single virus family (Tyrrell et al., 1978; ter Meulen et al., 1981). There are at least 3 subgroups of Coronaviridae, based on their mutual lack of antigenic cross-reactivity (Pedersen et al., 1978; Sturman and Holmes, 1983). Avian coronaviruses, such as infectious bronchitis virus (IBV), do not share antigenic determinants with mammalian coronaviruses. Mammalian coronaviruses fall into two distinct groups: one includes mouse hepatitis virus (MHV), bovine Coronavirus (BCV), hemagglutinating encephalomyelitis virus of swine (HEV) and human respiratory Coronavirus OC43; and a second includes human respiratory Coronavirus 229E, transmissible gastroenteritis virus of swine (TGEV), canine Coronavirus (CCV), and feline infectious Peritonitis virus (FIP). There are also coronaviruses antigenically unrelated to these three major subgroups, such as porcine enteropathic Coronavirus, CV777 (Pensaert et al., 1981).