One of the major challenges in radioimmunotherapy is the specific delivery of radioisotopes to tumor cells while minimizing normal tissue radiation. In this respect, the application of two-step pretargeting schemes generally leads to more favorable tumor to normal tissue uptake ratios than direct administration of radioimmunoconjugates. In this study, we present the specific hybridization of complementary DNA fragments as a novel recognition mechanism in pretargeting. Briefly, our strategy involves first administration of antibody-DNA conjugate, followed by targeting with radiolabeled complementary DNA (antisense DNA). Complementary oligodeoxynucleotides (14-mers, Tm = 57 degrees C), in which part of the phosphodiesters has been replaced by methylphosphonates (to ensure stability against nucleases), were prepared on a DNA synthesizer. The oligonucleotides were further derivatized via a uridine moiety at their 5'-end in such a way that radiolabeling or conjugation with antibodies could be accomplished. Both a murine IgG (anti-hCG) and the human anti-tumor IgM 16.88 were conjugated with one to three oligonucleotides via the heterobifunctional cross-linker SMCC. Incubation of these immunoconjugates with the radiolabeled antisense DNA revealed specific hybridization with the antibody-linked oligonucleotides. Antigen binding studies performed with antigen-coated matrices showed that the immunoreactivity of the antibody-DNA conjugate is preserved. Moreover, it is demonstrated that the radiolabeled DNA is still capable of hybridizing selectively with the oligonucleotides of the immunoconjugate, when the latter is bound to its antigen.