The present invention relates to antagonists or inhibitors, which bind selectively to RANKL/OPGbp and regulate the interaction between RANKL/OPGbp and RANK/OPG. In particular, the present invention relates to an antibody or antigen binding domain, fragment or derivative thereof, immunoreactive with a RANKL/OPGbppeptide for use in the treatment, prevention or alleviation of male infertility or reduced male fertility such as oligospermia or azospermia.