Immunosterilization is an attractive alternative to surgical castration. Gonadotropin-releasing hormone (GnRH) controls the production of the gonadotropins thereby having an orchestrating effect on the reproductive hormone cascade and spermatogenesis. Induction of neutralizing antibody can abrogate the effect of the hormone. Current GnRH-based vaccines often require strong adjuvants and/or multiple injections of the vaccines to overcome variability in the response. Heat shock proteins (hsp) have been used as carrier molecules because of their powerful intrinsic ability to enhance an immune response to associated antigens. A GnRH-analogue, GnRH-d6-Lys, was conjugated to recombinant Mycobacterium tuberculosis hsp70. Male BALB/c mice were immunized i.p. with GnRH-hsp70 in the mild adjuvant Ribi or in incomplete Freund's adjuvant (IFA). The initial immunizations were done on pre-pubertal 3-week-old mice, with boosts at 5 and 8 weeks of age. The mice were killed at 10 weeks of age and GnRH-specific antibodies and serum testosterone levels measured. All the immunized mice mounted GnRH-specific antibody responses, with no difference in the mice immunized with GnRH-hsp70/Ribi or with GnRH-hsp70/IFA. There was substantial atrophy of the urogenital complex and significantly (P < 0.0005) reduced levels of testosterone-dependent testicular relaxin-like factor mRNA expression. Mice immunized with GnRH-hsp70/Ribi showed substantially reduced (P < 0.001) serum testosterone levels. These results indicate that hsp70 may serve as a particularly advantageous carrier for GnRH-based vaccines.