Breast cancer cells express an array of weakly immunogenic tumor-associated antigens (TAAs). Under appropriate circumstances, immunity to breast cancer can be induced, with potential benefits for patients with the disease. Here, we report a new cell-based vaccination strategy resulting in enhanced immunity to breast cancer in tumor-bearing mice. The strategy was designed to enrich the vaccine for highly immunogenic cells. The vaccine was prepared by transfer of unfractionated complementary DNA (cDNA) derived from a highly malignant breast neoplasm that arose spontaneously in a C3H/He mouse (SB5b) into an immunogenic fibroblast cell line. As the transferred cDNA spontaneously integrates into the genome of the recipient cells and is replicated as the cells divide, sufficient DNA to prepare the vaccine could be obtained from as few as 107 cells (4-mm tumor). Because only a small proportion of the transfected cell population was expected to have incorporated genes responsible for inducing immunity to the breast cancer, we devised a novel approach designed to enrich the transfected cell population for cells that induced immunity to the neoplasm. Aliquots of the transfected population were divided into small pools (initial inoculum=4×105). Afterward, the cell number from each pool was allowed to expand in vitro. Pools containing greater numbers of immunogenic cells (identified by 2 independent assays) were subdivided for additional rounds of immune selection. Enhanced immunity to the neoplasm was detected in tumor-bearing mice treated solely by immunization with the enriched cell population. The immunity, mediated by CD8+ T cells, was sufficient to prolong the survival of mice with established breast cancer.