In the present work, mutual interaction of melittin, a pore forming hemolytic toxin from bee venom, and vitamin D-2, an antioxidant steroid, with dipalmitoyl phosphatidylcholine (DPPC) multilamellar liposomes has been investigated. Turbidity and Fourier transform infrared (FTIR) spectroscopic measurements, in combination with thermodynamic calculations, were used to monitor the modulating effect of vitamin D-2 on a melittin-DPPC membrane system. The results indicate that melittin on its own decreases the main phase transition to lower temperatures and also dramatically decreases the stability of the membrane. It has an overall disordering effect on the phospholipid membrane structures. Inclusion of vitamin D-2 at low concentrations (3, 6 mol%) into melittin containing DPPC liposomes slightly shifts the main phase transition to lower temperatures. High concentration of vitamin D-2 (9 mol%) has a more dramatic effect in shifting the main phase transition to lower temperature. It also causes a significant broadening in the phase transition curve. The present study also demonstrates that, with the addition of vitamin D-2 into melittin-DPPC system, absorbance value in turbidity study and the frequency of the CH, stretching band in FTIR study changes in a manner that are consistent with a reduction in the membrane perturbing effect of melittin on DPPC liposomes. Vitamin D-2 diminishes the disordering effect of melittin on DPPC lipids and produces a more ordered membrane system. These results were confirmed with thermodynamic calculations. (C) 2000 Elsevier Science B.V. All rights reserved.