Endemic Balkan nephropathy (EBN) is a kidney disease of unknown etiology limited to Bulgaria, Rumania and former Yugoslavia. Primary kidney tissue cultures were established as explants from tissue obtained at operations from 5 EBN patients with urinary tract tumors. Four out of the five biopsy specimens on extended culture incubation at 33°C yielded a coronavirus virus (EBNV) which was cytopathogenic for human fibroblast and Vero cells. In cells inoculated with EBNV, cytoplasmic immunofluorescence was found using antisera for human coronaviruses OC43 and 229E as well as the porcine transmissible gastroenteric virus and avian (chicken) bronchitis virus. In neutralization tests, EBNV failed to react with antisera to these viruses. Using hyperimmune serum raised with EBNV, positive cytoplasmic immunofluorescence was seen with cells infected with OC43, 229E, TGV and significantly with the kidney tissue of the biopsy specimens from the EBN patients. A screen for neutralizing antibody using the EBN virus revealed that 87.2% of EBN patients on dialysis were positive, also 74% of people from an endemic area were also positive, while only 13.5% from outside were positive. It is suggested that a coronavirus is involved in the etiology of the disease and that humans are an incidental host of a coronavirus zoonosis.