Enormous strides in cancer immunology have been made during the past decade. This is largely due to the development of methodologies capable of defining the antigenic targets on cancer cells that elicit a host immune response. The molecular cloning of antigens recognized by cytotoxic T cells by Boon and his colleagues has provided a growing list of tumor antigens, particularly for melanoma, that allows detailed monitoring of T cell responses to these antigens and offers promising targets for cancer vaccine development. An alternative new method, SEREX, for the serological identification of cancer antigens has been developed by Pfreundschuh and his colleagues. SEREX can be applied to all types of cancer including breast cancer that have been unapproachable by using cytotoxic T cells and thus offers an opportunity to define a vast range of cancer antigens. Toward thedevelopment of a vaccine for breast cancer, we have begun using SEREX to study breast cancer and have identified a few promising cancer antigens. Each antigen is now being critically evaluated as a possible vaccine target.