The mechanism by which cell suspensions are agglutinated by plant lectins remains obscure. The agglutination of a particular cell line in the presence of a specific plant lectin probably depends on several factors including the number and valence of lectin molecules bound to the cell surface, the mobility of receptor molecules in the membrane, the surface morphology and charge and the metabolic state of the cell1-6. The assay system used to assess cell agglutination also seems to be important, since many laboratories studying the same agglutination reaction have reported dissimilar or contradictory results. To provide further information on the molecular mechanism of agglutination we have begun a systematic study on the aggregation of human red cells by the lectin concanavalin A (Con A). By using an adaptation of our previously described aggregation assay which provides a continuous measure of both the rate and degree of intercellular adhesion in the presence of controlled shear forces, we have found that the agglutination of erythrocytes by Con A can be resolved into three stages, two of which are observed only if the system is exposed to shear.