The present invention relates to methods which utilize anti-idiotypic antibodies, or fragments thereof, for tumor immunotherapy or immunoprophylaxis. Monoclonal anti-idiotypic antibodies which recognize an idiotype present on a second antibody or on a T lymphocyte or on an immune suppressor factor which is directed against a defined tumor antigen, can be used for immunization against a tumor, for immune anti-tumor activation or inhibition of suppression, or for in vitro activation of lymphocytes to be used in adoptive immunotherapy. The anti-idiotypic antibodies, or fragments thereof, can also be used to monitor anti-antibody induction in patients undergoing passive immunization to a tumor antigen by administration of anti-tumor antibody. In another embodiment, administration of T lymphocytes which express an idiotype directed against a defined tumor antigen can be used to transfer delayed-type hypersensitivity to the tumor. In another method of the invention, the induction of anti-idiotypic antibodies in vivo by administration of anti-tumor antibody or immune cells or factors exhibiting an anti-tumor idiotype can be therapeutically valuable.