Proteins intended for treatment of bone diseases should ideally exhibit a high bone affinity, so that they are preferentially deposited to bones after systemic administration. This can be achieved by combining molecules having a high affinity to bone with the proteins. Bisphosphonates (BPs) are chemical analogs of pyrophosphate that possess exceptional bone mineral affinity. To this end, we synthesized a novel BP, 3,5-di(ethylamino-2,2-bisphosphono)benzoic acid (6), which contains two BP moieties on a single molecule, unlike conventional BPs that contain one BP moiety per molecule. 6 was then conjugated to two model proteins, bovine serum albumin and nonspecific bovine immunoglobulin G by the carbodiimide chemistry. By varying the reagent concentrations, the conjugation efficiency (i.e., number of 6 per protein) was readily controlled under the experimental conditions. The protein-6 conjugates exhibited an in vitro mineral affinity that was proportional to the number of conjugated 6. The 6-conjugates of both bovine serum albumin and immunoglobulin G were found to be bone seeking in rats, based on the increased concentration of 6-conjugated proteins in bone tissue after intravenous administration. We conclude that the novel BP synthesized (6) can serve as a carrier for bone delivery while reducing the extent of protein modification necessary for bone targeting.