It has been reported that kanamycin lowers osmotic fragility and mechanical fragility by ultrasonification of human red cells. In the present study, we therefore investigated effects of Ca⁺⁺, kanendomycin and kanamycin on stearic acid monolayer using a langmuir trough and found fellowing results 1) Ca⁺⁺ added to the subphase condensed stearic acid monolayer, confirming earlier reports. 2) Likewise, kanendomycin and kanamycin condensed the stearic acid monolayer, the effects being dose dependent. The possible modes of action of kanamycin and kanendomycin on biological membrane were discussed and compared with those of Ca⁺⁺ and local anesthetics