The biological activity of a lectin is influenced by its quaternary structure. Viscumin is special among the family members of toxic AB-type plant lectins, because it triggers mitogenicity, toxicity, and agglutination. Its activity profile is dependent on the concentration, motivating a thorough inspection of the status of quaternary structure. Over a broad range of protein concentrations (0.01-25 mg/mL), viscumin occurs as a dimer. At high concentrations, the solutions exhibited nonideality, self-association, and polydispersity in sedimentation equilibrium and velocity experiments caused by irreversible aggregation. Calculation of viscumin's overall shape based on sedimentation velocity data resulted in an elongated dimer form resembling that of crystallized agglutinin. Appearance of monomers was restricted to concentrations in the submicrogram/mL level, as demonstrated by fast protein liquid chromatography gel-filtration analysis. To shift the equilibrium to the monomer for comparative cell biological assays, we performed chemical modification under conditions protecting the lectin activity. Citraconylation was effective to destabilize the dimer. Binding studies by fluorescence-activated cell scan analysis revealed a reduction in cell association upon modification and a tendency for increased sensitivity towards haptenic inhibitors at microg/mL concentrations. Nonetheless, growth inhibition continued to be potent for the ricin-like monomer despite reduced extent of binding. Occurrence of a concentration-dependent monomer/dimer equilibrium appears to achieve the same objectives as the development of two separate protein entities in Ricinus communis, an alternative strategy to emergence of a monomeric toxin, and cell cross-linking dimeric agglutinin.