Abstract Ricinus communis invertase is activated by a number of macromolecules including proteins and lectins. This invertase is a glycoprotein containing ca 45% of sugars in the molecule. The activation by lectins is produced through the formation of a complex with the invertase sugars. The activation by lectins is always greater than that caused by proteins, ranging from 44 to 90%. However a part of the lectin links to the invertase by a different mechanism involving protein-protein interactions. This part appears to be responsible for a 30% activation. The differences in sugar specificities of the activating lectins suggest a complex sugar composition of the invertase. Two lectins, R. communis agglutinin and ricin, described in seeds of the same plant, are activators of R. communis invertase.