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In this study photo-control of the non-biospecific interaction between endothelial cell membranes and photo-cation generatable water-soluble polymers were examined. The water-soluble polymers contained triphenylmethane leucohydroxide (malachite green) groups (contents: 0.4 and 1.6 mol%), which dissociate into triphenylmethyl cations and counter hydroxide ions upon ultraviolet light (UV) irradiation, and were prepared by free radical copolymerization of diphenyl(4-vinylphenyl)methane leucohydroxide and acrylamide. The nature and magnitude of the interaction was quantitatively assessed by direct luminescence measurement of the intracellular calcium ion concentration using a calcium-sensitive photoprotein, aequorin. When a PBS buffer of the photoreactive copolymers were added, prior to UV irradiation, to a PBS suspension of cultured bovine endothelial cells loaded with aequorin, no detectable elevation of Ca2+ was measured. In contrast, cationic copolymers, derived from the photoreactive copolymers after UV irradiation at a wavelength of 290<λ<410 nm, induced an immediate transient increase in the cytosolic free Ca2+ concentration due to a Ca2+ inflow from the extracellular space into the cells, which may be due to non-biospecific transmembrane stimulation. Longer UV irradiation exposures of the copolymers and higher concentrations of the polymers, with higher contents of the photodissociable group, resulted in more Ca2+ inflow with little cellular damage. The photo-cation generatable copolymers developed here made possible to control the non-biospecific interaction with endothelial cell membranes by UV irradiation condition, and composition and amount of the copolymer.