A monovalent form of concanavalin A (m-Con A) has been prepared to determine the importance of valence for human lymphocyte surface binding and subsequent lymphocyte stimulation as measured by blast transformation and cytotoxicity. Concanavalin A (Con A) was fragmented by a proteolytic process and the m-Con A) derivative was isolated by elution with an ascending D-glucose gradient on a Sephadex G-200 column. The molecular weight of m-Con A was 18,000 by sodium dodecyl sulfate-polyacrylamide electrophoresis. Equilibrium dialysis with alpha-methyl D-glucoside and subsequent Scatchard plot analysis revealed an association constant (Ka) of 1.2 X 10(3) liters/mol and a valence of 1.1. Incubation of lymphocytes with 125I-labeled m-Con A demonstrated surface binding at 1.21 X 10(6) molecules per cell, which was comparable to the binding of [3H] Con A (1.02 X 10(6) molecules per cell). However, in contrast to the intact lectin, m-Con A had a markedly reduced capacity to agglutinate rabbit erythrocytes and human lymphocytes and did not stimulate lymphocyte blast transformation or cytotoxicity at 1 and 10 mug/ml. Finally, pretreatment of lymphocytes with m-Con A blocked blast transformation induced by Con A. These observations demonstrate that m-Con A binds to lymphocyte surface receptors but does not stimulate blast transformation or cytotoxicity, suggesting that Con A must bridge binding sites on the lymphocyte surface to induce lymphocyte activation.