The present invention provides a method for making uncapped cysteine protein preparations, including uncapped engineered cysteine antibody preparations. The methods include, inter alia, contacting a reducing agent with engineered cysteine antibody molecules, each of the antibody molecules having at least one capped engineered cysteine residue and at least one interchain disulfide bond and reacting the reducing agent with the antibody molecules under conditions sufficient to uncap engineered cysteine residues and form cap byproducts. The method also includes removing the cap byproduct during the reduction reaction. Substantially all of the interchain disulfide bonds present in the antibody molecules prior to reduction are retained following reduction. Antibody conjugates and methods for preparing antibody conjugates using uncapped antibody preparations are also described.
