The interaction of the chemical carcinogen benzo[a]pyrene (BaP) with phosphatidylcholine membranes has been investigated by using various physical techniques. Differential scanning calorimetry showed that BaP, at concentrations as low as 2 mol% in mixtures with dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine, and distearoylphosphatidylcholine, abolished the pretransition and broadened and shifted to lower temperatures the main gel-to-liquid crystalline phase transition. The effect was stronger as the lipid acyl chain length decreased. Infrared spectroscopy indicated that inclusion of BaP had a strong influence on the carbonyl (C=O) stretching band. Fitting of this band resulted in the presence of three components in the case of samples containing BaP, centered at 1,742, 1,727, and 1,704 cm(-1) (only two components, at 1,742 and 1,727 cm(-1), were observed for pure DMPC). The component at 1,704 cm(-1) corresponded to a disolvate and strongly supported the notion that inclusion of BaP increases hydration of the bilayer C=O. Small-angle x-ray diffraction showed that incorporation of 10 mol% BaP into DMPC increased bilayer thickness from 66.7 to 71 A in the gel state and from 58.9 to 60.9 A in the fluid phase. It is proposed that, when BaP is incorporated into a phospholipid bilayer, it locates in the most apolar region of the phospholipid palisade, resulting in expansion and swelling of the membrane. These two effects will facilitate water penetration into the polar region of the membrane, as observed by the increased hydration of the C=O groups, and thus perturb both membrane integrity and functionality.