The heptadecapeptides bombolitin I and bombolitin III are two members of a series of biologically active peptides postulated to be membrane active. In order to understand the effects of the membrane on the secondary structure of the peptides, we have carried out the conformational characterization of bombolitins I and III in the presence of SDS micelles using circular dichroism, nuclear magnetic resonance, and computer simulations. The characteristic bands in the circular dichroism spectra indicate an alpha-helix content of approximately 60% in bombolitin III and 70% in bombolitin I. The observation of NOE's quite distinctive for such secondary structure strongly supports the CD results. The conformational preferences of the two bombolitins derived from CD and NMR were then energetically refined with molecular dynamics simulations. The results from the spectroscopic examination were utilized as input for the simulations, the CD results for generation of the initial structure, and the NOE's as constraints during the simulations. The results from the different techniques employed are in complete agreement.