A hybrid peptide of cecropin A and melittin was investigated by two-dimensional 1H-NMR at pH 5.8 in aqueous solution with 30% (by vol.) hexafluoroisopropanol. The peptide contains 26 amino acids, is a combination of the first 13 residues of each of the two parent peptides, CA(1-13)M(1-13) identical to CAM(1-26) and has an amidated C terminal. This peptide was recently synthesized [Boman, H.G., Wade, D., Boman, I.A., Wåhlin, B. & Merrifield, R.B. (1989) FEBS Lett. 259, 103-106] and shown to have strong antibacterial activity but to be harmless towards erythrocytes. All resonances of the main chain and side chain beta-protons are assigned except for those of the N-terminal lysine. Several medium range NOE connectivities were observed showing two separated alpha-helices, involving residues 4-12 and 16-26. The JNH alpha-coupling constants in these sections support the conclusion. From the exchange rates of the NH protons it is concluded that the alpha-helix of residues 16-26 is much more stable than the other helix. The circular dichroism data indicates about 30% less alpha-helix character than the NMR data. A reduced contribution to the ellipticity from the unstable helix is suggested. The chemical-shift differences between the two parts of the hybrid and the respective parent peptides are larger for the cecropin part than for the melittin part. For the latter, residues 17-26 of the hybrid are proposed to have a secondary structure very similar to that of residues 4-13 of melittin.(ABSTRACT TRUNCATED AT 250 WORDS)