The beta-subunit of human chorionic gonadotropin (hCG) conjugated to tetanus toxoid is being investigated as a vaccine for human fertility control. Initial clinical trials indicated that the level of antibody response induced by such an immunogen was not always sufficient to prevent pregnancy. Therefore, efforts are being made to evaluate new carriers for the beta-subunit and to select adjuvants to yield a more efficient vaccine. In the present report, we demonstrate that conjugates of the beta-subunit of hCG with muramyl dipeptide (MDP), or its nonpyrogenic derivative murabutide, may have potential as an effective antipregnancy vaccine. The copolymer of beta hCG and MDP administered with Al(OH)3 to mice induced a high anti-beta hCG response, better than that induced by the conjugate of beta hCG to tetanus toxoid given with Al(OH)3. Moreover, the antibodies induced by such an immunogen were competent for neutralizing the biological activity of hCG in vivo. Even more interesting, a copolymer of beta hCG and of murabutide induced high levels of biologically active antibodies. This immunogen may represent a promising candidate for the development of an efficient vaccine for human fertility control.