Rat X rat hybridomas secreting monoclonal anti-idiotypic antibodies have been prepared from Hooded rats immunized with two tumour-reactive, syngeneic monoclonal antibodies 11/160 and M10/76 (specific, respectively, for the Hooded rat sarcomata HSN and MC24). The hybridomas were selected on the basis that the secreted antibodies competed with antigen for binding to the immunizing idiotype. One monoclonal anti-idiotype (HIM/1/230, gamma 2a isotype) that recognizes an antigen-binding site idiotope of antibody 11/160 has been found to substitute for antigen. Hooded rats vaccinated by three challenges with HIM/1/230 produce serum Ab3 that is indistinguishable in antigen specificity from the 11/160 Ab1, and show reduced tumour take following an i.v. challenge with 10(6) HSN cells. The response to vaccination with anti-idiotype was both qualitatively and quantitatively dependent on the mode of immunization. High titre 11/160-like Ab3 was generated only when the vaccine contained Freund's adjuvant, whereas resistance to tumour challenge was found only in animals vaccinated with anti-idiotype in the absence of adjuvant.