Summary—Dog embryo kidney cells transformed by the human cytomegalovirus (HCMV) were obtained after non‐permissive infection or transfection with viral DNA digested by restriction endonucleaseEcoR I. The transformed cells, growing rapidly and showing an unlimited division potential, could use medium with only 2% serum for growth, contained nuclear virus antigens, and formed small colonies (<0.2 mm) in agarose. From 40 mice inoculated with transformed canine cells, only one eventually developed a tumor. Results indicate that dog cells are immortalized but not tumorigenically transformed by the human cytomegalovi