Anti-dinitrophenyl IgE secreting hybridoma B 53 cells may be rejected when injected subcutaneously in BALB/c mice. These mice are immune as they withstand without any ill effect the intraperitoneal injection of LD100 B 53 cells. Sera from mice which rejected the tumor have cytotoxic antibodies against the hybridoma, as shown by in vitro tests, but serum cannot transfer immunity to naive BALB/c mice against hybridoma B 53. Spleen cells from mice which have rejected the tumor might transfer immunity against B 53 hybridoma, and with Winn tests it has been shown that these spleen cells are very effective against the B 53 cells and also against the myeloma cells which were used for the fusion to construct the B 53 hybridoma. Subcutaneously injected B 53 cells not only produce anti-DNP IgE secreting tumors, but often also metastasize to spleen, and they are sometimes detected in the circulating blood. Mice with splenic metastasis or with detectable circulating B 53 cells generally die. However, we did observe one mouse with splenic metastasis which successfully rejected the tumor and became immune to B 53 cells.