Abstract Fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene was used to study the effects of polylysine on the thermotropic behavior of dipalmitoylphosphatidylglycerol (DPPG) bilayers. The molecular weight of the polypeptide was shown to be a key parameter. Long polylysine ( mol. wt. ⩾ 60000 ) induces a lateral phase separation in three domains for lipid to lysyl residue ratios ( R i ) greater than unity and a shift of +4.0 Cdeg of the single gel-fluid transition is observed when there is an excess of lysine. With short polylysine (mol. wt. approx. 4000), no domain formation was observed and the transition temperature shift was much smaller. Polylysine with mol. wt. 17000 yielded a very complex lipidic behavior, with a triphasic transition for R i > 1 , another triphasic transition for 1 > R i > 0.1 , and finally, a 4.0 Cdeg positively shifted one-step transition at higher lysine concentration. These divergencies in effects for different degrees of polymerization of the polypeptide are believed to be related to structural parameters. Multilamellar liposomes and small unilamellar vesicles react in the same way to polylysine, whatever its degree of polymerization. However, the use of the latter has permitted the observation of the gradual destruction of unilamellar structure when going from the pure lipid to R i = 1 . The optical d- and l-isomers gave identical results.