The invention provides methods for preparing an antibody-active agent conjugate. The conjugate comprises an antibody, a cysteine residue of an amino acid motif that can be recognized by an isoprenoid transferase located at or after the carboxy-terminus of the antibody, an isoprenoid unit operably linked to the cysteine residue, and an active agent. The invention also provides a composition comprising the antibody-active agent conjugate prepared by the methods.
