Oncolytic vaccinia virus is an engineered vaccinia virus that selectively destroys cancer cells and induces tumor immune response. Oncolytic vaccinia expressing mouse GM-CSF showed cytotoxic activity against various kinds of cancer cells when oncolytic vaccinia virus expressing human GM-CSF and mouse GM-CSF is intravenously administered in the mouse CT26 colon tumor model. Cancer cells treated with isolated immunoglobulin G from the serum with complement showed these cytotoxic activity and complement observed dose-dependent cytotoxic effect. These results suggest that oncolytic vaccinia virus expressing mouse GM-CSF can increase oncolytic vaccinia virus by inducing anticancer antibody in a mouse tumor model. Further studies are needed on antitumor immunity of GM-CSF.