The effects of phytohemagglutinin (PHA) and illumination on the surface charge densities and 90degrees light scattering properties of pea and Chlamydomonas reinhardtii thylakoids were investigated. The electrophoretic mobility (EPM) of pea thylakoids decreased after treatment by various concentrations of PHA at ionic strengths of I=0.01 and I=0.02, while that of C. reinhardtii thylakoids remained stable except for a drop after treatment by PHA at a concentration of 6 ng/mL in a medium with an ionic strength of I=0.01. Illumination did not influence the EPM of untreated thylakoids. However, if the EPM of thylakoids had been retarded by pretreatment with PHA, light exposure stimulated a recovery of the reduced negative surface charge density up to at least the initial values. In addition to reducing EPM, PHA also induced a decrease of the basal light scattering property of pea thylakoids, which is an indicator of thylakoid aggregation. The physiological role of the membrane surface charges of thylakoid particles in lectin regulated processes of thylakoid stacking and activity is discussed.