Low molecular weight random copolymers hearing protonated primary airline groups and hydrophobic alkyl groups in the side chains were synthesized and their activities against E. coli, S. aureus, human red blood cells, and human epithelial carcinoma cells (HEp-2) were quantified. The mole fraction of alkyl side chains in the copolymers (f(alkyl)) and the length of the alkyl chains were major determinants of the activities, Against E. coli cells, activity was diminished as f(alkyl) was increased from 0 to about 0.2, but was then enhanced dramatically as f(alkyl) was increased further. Activity against S. aureus was diminished continually with increasing f(alkyl). The cytotoxicity to human epithelial carcinoma cells also decreased with increasing f(alkyl), Conversely, hemolytic activity showed monotonic enhancement with increasing f(alkyl). The cationic homopolymer (f(alkyl) = 0) completely inhibited S. aureus growth at 3 mu M (10.2 mu g/mL) and completely inhibited metabolic activity in HEp-2 cells at 10 mu M (34 mu g/mL), although it did not induce any detectable hemolysis up to 645 mu M (2000 mu g/mL). Polymer-induced dye leakage from liposomes provided a biophysical basis for understanding the factors which modulate the polymer-membrane interactions. Disruption of Zwitterionic POPC vesicles induced by the copolymers was enhanced as f(alkyl) increased, following trends similar to the hemolytic activity data. The ability of the polymers to permeabilize vesicles of POPE/POPG and DOPG/Lysyl-DOPG/CL displayed trends similar to trends in their activities against E. coli and S. aureus, respectively. This was interpreted as evidence that the antimicrobial mechanism employed by the polymers involves disruption of bacterial cell membranes. An investigation of leakage kinetics revealed that the cationic homopolymer induced a gradual release of contents from POPE/POPG and DOPG/Lysyl-DOPG/CL vesicles, while the more hydrophobic copolymers induced rapid dye efflux. The results are interpreted as evidence that the cationic homopolymer and hydrophobic copolymers in this study exert their antimicrobial action by fundamentally different mechanisms of membrane disruption.