General immunostimulants (adjuvants) are essential for generating immunity to many antigens. In bacterial infections, adjuvants are provided by components of the microorganism, e.g., lipopolysaccharide. However, it is unclear what provides the adjuvant effect for immune responses that are generated to tumors and many viruses. Here we show that cell injury and death of tumor or even normal cells provide a potent adjuvant effect for the stimulation of cytotoxic T lymphocyte responses. This adjuvant activity is constitutively present in the cytoplasm of cells and is increased in the cytoplasm of cells dying by apoptosis. The release of these components stimulates immune responses both locally and at a distance, and provides a simple mechanism to alert the immune system to potential danger in almost all pathological situations.