The effect of the anticancer drug bleomycin on acyl chain order of KB cell membranes was examined by electron paramagnetic resonance and fluorescence polarization spectroscopies using respectively, the 5-doxyl stearic acid spin probe and the 1,6-diphenyl-1,3,5-hexatriene and the 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene fluorescent probes. Measurements of the order parameter, S, by the two techniques showed a perturbation of the plasma membrane fluidity with bleomycin-iron, while no effect was observed with bleomycin or iron alone. A kinetic study of the location of the 1,6-diphenyl-1,3,5-hexatriene fluorescent probe into the cell was followed by fluorescence microscopy. Lipid peroxidation measurements were also performed using isolated unsaturated lipids, intact cells, or isolated plasma membranes whose purity was checked by electronic microscopy. These membrane perturbation effects not observed with bleomycin-iron in the presence of a hydroxyl radical scavenger, dimethyl thiourea, or a chelating agent, desferrioxamine, were correlated with the ability of the complex to generate highly reactive oxygen species.