Coronaviruses of human origin have emerged as probable important etiologic agents of acute upper respiratory tract illnesses in adults; their role in the etiology of respiratory illnesses of childhood is less certain. The first coronavirus of human origin, B814, was described in 1965 and since that time only 31 additional strains have been recovered. Ten of these were originally recovered in human embryonic tracheal organ culturesonly, and the remainder in monolayer cell cultures. All of the latter group were antigenically related to a prototype strain designated 229e. As a result of the fastidious growth requirements of these agents, most of the knowledge concerning the clinical syndromes associated with them has come from chajlenge studies in volunteers and seroepidemiologic investigations. Coronaviruses have been shown to be associated with a substantial number of adult respiratory illnesses especially during certain periods when rhinovirus infections are infrequent. Progress in propagating coronaviruses and in detecting coronavirus infections has been achieved recently: viruses similar to 229E were successfully recovered in human embryonic intestine cell cultures from patients with upper respiratory tract illnesses; 30C strains (B814, LP and EVS) were recovered in L132 cell cultures from infectious nasal washings; OC43 (and OC38) viruses which were originally recovered in OC and later adapted to grow in suckling mice were not only shown to directly hemagglutinate various erythrocytes but were also adapted to grow in MK cell cultures; OC43 virus was also found to induce hemadsorption of rat and mouse erythrocytes in certain cell cultures; and a coronavirus strain was detected by immune electron microscopy. Coronaviruses are also associated with certain diseases in animals.