Tissue samples were removed at necropsy from five American bison (Bison bison) with clinical signs of a disease resembling malignant catarrhal fever (MCF). Using cell associated virus techniques, attempts were made to isolate viruses from these tissues by culturing them directly or by co-culture with bovine fetal cells. Among the viruses isolated was one which was syncytiogenic and multiplied in bovine fetal spleen cells and remained highly cell-associated. The presence of reverse transcriptase activity indicated that it was a retrovirus. Also, it had antigenic cross activity with bovine syncytial virus, but not with bovine leukemia or bovine maedi-like retroviruses. We do not attribute a direct causative role of this retrovirus to MCF, but indirect relationships are possible.