The antibacterial activity of chitosan modified with the thiol-containing 2-iminothiolane HCl (TC-IMI) and N-acetyl- L -cysteine (TC-NAC) was studied by Langmuir film balance technique using a dipalmitoylphos- phatidylglycerol (DPPG) monolayer bacterial membrane model. The interactions of the biopolymer with the membrane model were assessed by monitoring the differences in the shape of the compression iso- therms recorded in the absence and presence of chitosan and thiolated conjugates in the subphase. A low molecularweightchitosan(15 kDa)shiftedthecompressionisothermsofDPPGmonolayerstowardslarger areas ( A 0,CS = 145 Å 2 ), confirming its membrane disturbance capacity. Further thiolation induced higher yield of expansion, more pronounced in the case of TC-IMI. The expansion of the monolayer increased sig- nificantly( A 0,TC-NAC = 150 Å 2 vs A 0,TC-IMI = 175 Å 2 )andtheelasticityatasurfacepressureof30 mN/m,typical for bio-membranes decreased to a greater extent C 1 s ; 30 TC NAC = 120 mN/m vs C 1 s ; 30 TC IMI = 87 mN/m) in presence of TC-IMI. Antibacterial tests against a Gram-negative Escherichia coli and a Gram-positive Staph- ylococcusaureus wereingoodagreementwiththesefindings,suggestingthatchitosanthiolatedwith2-imi- nothiolane HCl acts as a bactericide disrupting the integrity of the bacterial cell membrane