Trastuzumab (Herceptin®) is an FDA-approved therapeutic antibody currently employed in the treatment of metastatic stages of breast cancer. Herein, we propose a simple, fast and cost-effective methodology to conjugate trastuzumab with 22-mer 5' thiol-modified oligonucleotides using a bifunctional crosslinker. The conjugates were successfully characterized by MALDI-ToF MS and SDS-PAGE, obviating the need for enzymatic digestion and difficult chromatographic separations. Furthermore, ELISA was performed to ensure that trastuzumab activity is not affected by oligonucleotide conjugation.