We investigated the effect of the antineoplastic drug doxorubicin on the order of the acyl chains in liquid-crystalline mixed bilayers consisting of dioleoylphosphatidylserine (DOPS) or -phosphatidic acid (DOPA), and dioleoylphosphatidylcholine (DOPC) or - hosphatidylethanolamine (DOPE). Previous 2HNMR studies on bilayers consisting of a single species of di[ 11,l 1-2H2]oleoyl-labeledp hospholipid showed that doxorubicin does not affect the acyl chain order of pure zwitterionic phospholipid but dramatically decreases the order of anionic phospholipid [de Wolf, F. A., et al. (1991) Biochim. Biophys. Acta 1096, 67-80]. In the present work, we studied mixed bilayers in which alternatively the anionicor the zwitterionic phospholipid component was 2H-labeled so as to monitor its individual acyl chain order. Doxorubicin decreased the order parameter of the mixed anionic and zwitterionic lipids by approximately the same amount and did not induce a clear segregation of the lipid components into extended, separate domains. The drug had a comparable disordering effect on mixed bilayers of unlabeled cardiolipin and 2H-labeled zwitterionic phospholipid, indicating the absence of extensive segregation also in that case. Upon addition of doxorubicin to bilayers consisting of 67 mol % DOPE and 33 mol 7% anionic phospholipid, a significant part of the lipid adopted the inverted hexagonal (HII) phase at 25 OC. This bilayer destabilization, which occurred only in mixtures of anionic phospholipid and sufficient amounts of DOPE, might be of physiological importance. Even upon formation of extended HII-phase domains, lipid segregation was not clearly detectable, since the relative distribution of *H-labeled anionic phospholipid and [2H] DOPE between the bilayer phase and HII phase was very similar. Our findings argue against a role of extensive anionic/zwitterionic lipid segregation in the mechanism of action and toxicity of doxorubicin.