The lectins fromDatura stramonium, Lycopersicon esculentum, andSolanum tuberosumare structurally related and possess a similar carbohydrate specificity, yet theDaturalectin is mitogenic for human lymphocytes while the other two are not. However, theDaturalectin was found to antagonize blast transformation induced by purified protein derivative (PPD), even within the concentration range at which it was optimally mitogenic on its own. The presence of a submitogenic concentration of the phorbol ester 12‐O‐tetradecanoyl‐phorbol‐13‐acetate (TPA) enormously enhanced the mitogenic activity ofDaturalectin. No such synergy was observed, however, between tomato lectin orS. tuberosumagglutinin (STA) and TPA, nor betweenDaturalectin and the calcium ionophor