The concept of immunologic responses against tumors is currently under intense scrutiny throughout the world. The evidence for the existence of tumor-specific transplantation antigens (TSTA) and specific immune reactions to them in experimental animals is overwhelming. The available data concerning human tumors are controversial. The reason for this is partially that antigens detectable on human tumors by in vitro assays have not been biologically characterized. In other words, we do not know if the antigens on human tumors are acting as the targets for immunologically mediated rejection...