The interaction of benzyltrimethyl- and dodecyltrimethylammonium bromides and chlorides with red blood cells and planar lipid membranes was studied. The aim of the work was to find whether the type of a counterion influence that interaction. The model membranes were pig erythrocytes and planar lipid membranes (BLM). A change of mechanical properties of these membranes was observed in the presence of the compounds studied. The measured parameters were a degree of hemolysis of erythrocytes and stability of BLM`s. The critical concentration of a compound was the measure of BLM stability.The lifetime of BLMs under such concentration of a compound did not exceed 3 min. It was found in each case that bromides influenced model membranes slightly stronger than the corresponding chlorides. A possible explanation of the results obtained is presented. Namely, it seems that the greater efficiency of bromides to destabilize model membranes in comparison with chlorides can be attributed to greater mobility and smaller radius of the hydrated bromide ion. This can be the reason why bromide anions modify surface potential of model membranes more efficiently, which in turn leads to more intensive interaction of bromides with such modified bilayer in comparison with their analog chlorides.