The mitogenic activity of soybean agglutinin was found to depend on the presence of lectin aggregates formed in lectin preparations stored in the lyophilized state. Such soybean agglutinin preparations gave maximal stimulation of untreated pig lymph node cells and neuraminidase-treated mouse spleen cells at relatively high concentrations, ranging from 100 to 2000 mug/ml. After separation into unaggregated (divalent) and polymeric (tetra-and multivalent) fractions, it was found that the unaggregated lectin did not stimulate the cells, while the tetravalent and multivalent fractions were active and gave maximal stimulation at a concentration of 10 mug/ml. These results suggest that soybean agglutinin must have at least four sugar binding sites in order to be able to stimulate lymphocytes.