This invention relates to a solid phase method of synthesizing biomolecule-drug conjugates. In particular, this invention relates to a solid phase method of synthesizing antibody-drug conjugates (ADCs). This invention also relates to intermediate methods of producing immobilized, chemically modified biomolecules, e.g. antibodies. The invention also relates to various uses of capture resins and to biomolecule-drug conjugates, intermediate products and compositions prepared by the methods of the invention.