Аннотация:The goal of this work is to comparatively study the side effect PDT on blood with the use of two Russian photosensitizers -- photohem and photosense. These effects were studied at in vitro and in vivo conditions. The technique of backscattering nephelometry was used to measure the parameters of aggregation kinetics and the strength of aggregates of whole blood erythrocytes. The conventional method of absorption spectrophotometry was used to monitor the level of erythrocytes hemolysis. It was shown that sensitization of blood leads to the rise of the characteristic time of Rouleax formation, of the strength of aggregates, and of the level of hemolysis. The revealed tendency to increased strength of aggregates and level of hemolysis as a result of photodynamic action can be considered as a side effect of PDT.