A coronavirus-like agent isolated from feces of a calf with diarrhea and attenuated by consecutive passage in a fetal bovine kidney cell line was characterized as a coronavirus. Negatively stained virions were approximately circular, had a mean diameter of 120 nm, and were covered with wide-spaced, petal-shaped projections about 20 nm long. Virions in ultrathin sections of infected cell monolayers had a mean diameter of 80 nm, lacked surface projections, and were found within cytoplasmic vesicles. Viral antigen was demonstrated by immunofluorescence microscopy to occur only in cytoplasm. Growth of the virus was not inhibited by 5-iodo-2'- deoxyuridine and actinomycin D. The virus was sensitive to ether, chloroform, deoxycholate, and heat treatment. However, thermosensitivity was stabilized in the presence of 1 M MgCl2; at pH 3, the virus was stable. Hemadsorption and hemagglutination were observed with erythrocytes of hamsters, mice, and rats but not with erythrocytes of cats, dogs, goats, sheep, cattle, horses, turkeys, chickens, guinea pigs, rabbits, geese, pigs, and man (type O). However, hemadsorption and hemagglutination were shown to be virus specific, since this could be inhibited by specific antiserum. Both infectivity and hemagglutinating activity were maximal at a particle density of 1.18 g/ml by sucrose density gradient centrifugation, indicating that hemagglutinin was part of the virion.
