Turbidity technique was employed to study the mutual interaction of melittin and vitamin D-2 With 1,2-dipalmitoyl phosphatidylcholine (DPPC) bilayers. Ln addition, the data obtained from the phase transition curve is used to calculate the main phase transition temperature, conformational stability and activation energy values by using two state transition thermodynamic approach. The results indicate that melittin decreases the main phase transition temperature and also dramatically decreases the stability. Melittin on its own has disordering effect on phospholipid membrane structure. At low concentrations (3, 6 mol %) vitamin D-2 does not alter the main phase transition temperature, whilst the main phase transition temperature significantly decreases upon addition of a high concentration of vitamin D-2 (12 mol %). Stability values of mutual interaction of vitamin D-2 with DPPC indicates that vitamin D-2 has an ordering effect. Moreover, the activation energy increases as vitamin D-2 concentration increases. (C) 1999 Elsevier Science B.V. All rights reserved.