Monoclonal antibody (mAb) therapy applications have been growing rapidly in recent years. Like other recombinant protein drugs, therapeutic mAb's need to be well characterized to ensure their structural and functional integrity. IgG mAb's are composed of two heavy and two light chains covalently linked by interchain disulfide bonds. Each domain of the heavy or light chain contains one additional disulfide bond. Native IgG mAb's, with completely formed disulfide bonds, should not bear any free sulfhydryl. This report describes detection and quantification of free sulfhydryl in recombinant mAb's produced in Chinese hamster ovary (CHO) cells using a fluorescent technique. The method utilizes the fluorescent probe N-(1-pyrenyl)maleimide (NPM). The purified mAb's appear to be homogeneous under native conditions with approximately 0.02 mol of free sulfhydryl per mole of protein. Upon denaturation, minor species related to the mAb's are observed on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and the free sulfhydryl level is determined to be approximately 0.1 mol/mol of protein. These results suggest that a small portion of these recombinant mAb's lack in intermolecular disulfide bonds but remain noncovalently associated under native conditions. The formation of the free sulfhydryl containing mAb species is likely to occur during the culture process and/or protein folding process in the endoplasmic reticulum (ER).