The interactions between chlorophyll a, and three cyclodextrins, hydroxypropyl-beta-cyclodextrin heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin and hydroxypropyl-gamma-cyclodextrin, were studied in aqueous solutions by means of absorption, emission and circular dichroism spectroscopy. Nanosecond laser flash photolysis and steady-state singlet oxygen generation experiments were performed to clarify the photoactivity of chlorophyll a in these systems. Moreover the photosensitizing activity of these complexes towards human leukemia T-lymphocytes (Jurkat cells) was tested and compared with that of the free sensitizer, chlorophyll a. The results obtained indicate that each cyclodextrin is able to carry the pigment in monomeric form inside of cells producing singlet oxygen.