A virulent bovine coronavirus isolate (newborn calf diarrheal) was adapted and serially passaged in an established diploid swine testicular cell line (ST cells). The same cells have been used to produce modified live porcine rotavirus and coronavirus vaccines that are federally licensed and sold worldwide. Growth of the bovine coronavirus resulted in cytopathic effect characterized by cellular stranding and subsequent cell lysis. Virus yields were relatively high in the ST cells and active replication was confirmed by immune electron microscopy and immunofluorescence. Adaptation of bovine coronavirus to a diploid swine cell line has not been previously reported. Different cell culture passage levels of bovine coronavirus were evaluated by oral inoculation of clean-catch, colostrum-deprived calves. A passage level of bovine coronavirus was identified that multiplied in the calf without the clinical signs of disease associated with virulent passages. The modified live bovine coronavirus vaccine remained safe and efficacious even after 5-backpassages in calves. Further efficacy studies have shown that the modified live bovine coronavirus vaccine significantly protected calves from highly virulent challenges with either winter dysentery or newborn calf diarrheal coronavirus isolates.