Monovalent dimers of concanavalin A (Con A) have been prepared by a combination of succinylation and photoaffinity labeling. Partial derivatization of native Con A using the photoaffinity label, p-azidophenyl-alpha-D-mannopyranoside, followed by affinity chromatography yielded a fraction that consisted of dimers with a single saccharide-binding site at pH 5. These monovalent dimers formed divalent tetramers at pH 7. In order to achieve a monovalent dimer at this pH, the divalent tetramers were succinylated by previously developed methods. Ultracentrifugation, equilibrium dialysis, and chromatographic experiments indicated that the resultant preparations consisted mainly of monovalent dimers which showed subunit exchange to yield about 15% divalent dimers after 12 hr at physiological pH. Freshly prepared material failed to agglutinate sheep erythrocytes at concentrations 500-fold higher than native tetravalent Con A. In addition, they showed saturating dose-response curves of mitogenic stimulation of mouse splenic lymphocytes. These curves resembled those of divalent succinyl-Con A but not those of the native molecule. Further development of methods for preparing stable monovalent derivatives of Con A should allow a refined analysis of the effects of lectin valence at the cell surface.