Provided is a method or the like for producing a composition exhibiting cytocidal activity. This method for producing a composition exhibiting cytocidal activity comprises: culturing malignant tumor-derived cells in a culture medium at least until the cell density reaches a level that does not pose a problem for transfer； replacing, after culturing, the culture medium with a physiological buffer salt solution； and recovering the physiological buffer salt solution after death of the malignant tumor-derived cells is observed morphologically in the physiological buffer salt solution.
