The present invention provides a method of inhibiting maturation of the gonads of a juvenile animal which comprises administering to said juvenile animal an immunologically active molecule (IAM) or a vector comprising nucleic acid encoding an immunologically active molecule, said IAM being specific for a target protein within the gonads and binding thereto e.g an antibody or causing an immune response against that target protein, and thereby inhibiting maturation of the gonads, as well as molecules of use in such methods.
