Fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to study the interaction of DDT with model and native membranes. DDT decreases the phase transition midpoint temperature (Tm) of liposomes reconstituted with dimyristoyl-, dipalmitoyl- and distearoylphosphatidylcholines (DMPC, DPPC and DSPC), and broadens the thermotropic profile of the transition. The effects of DDT are concentration dependent and are more pronounced in bilayers of short-chain lipids, e.g., DMPC. The insecticide fails to alter DPH polarization in the fluid phase of the above lipids. Similar effects were observed in binary mixtures of DMPC plus DPPC. Furthermore, DDT alters the single broad transition of the equimolar mixture of DMPC plus DSPC into a biphasic transition. The lower temperature component has a midpoint at 25 degrees C, i.e., a value close to the Tm of DMPC. DDT inhibits to some extent the cholesterol-induced ordering in DMPC bilayers and high cholesterol concentrations (greater than or equal to 30 mol%) do not prevent insecticide interaction, conversely to the effect observed for lindane (Antunes-Madeira, M.C. and Madeira, V.M.C. (1989) Biochim. Biophys. Acta 982, 161-166). Apparently, the bilayer order is not disturbed by DDT in fluid native membranes of mitochondria and sarcoplasmic reticulum, but moderate disordering effects are noticed in membranes enriched in cholesterol, namely, brain microsomes and erythrocytes.