LY146032 is one of the A21978C family of calcium-dependent antibiotics. This paper reports on its interactions with membranes as studied by its intrinsic fluorescence. The Trp residue was found to have a low fluorescence yield because of Förster-type energy transfer to the kynurenine residue (Kyn) (epsilon = 5000 at 364 nm). However, the Kyn fluorescence (lambda max = 465 nm in H2O) was a sensitive probe of the membrane interactions, and it was used in steady-state fluorescence measurements including fluorescence polarization anisotropy. Initial binding of the peptide to phospholipid vesicles occurs in calcium-free solutions. When calcium is added, the resulting 10-fold fluorescent enhancement and 15-nm blue shift show that it causes the antibiotic to penetrate further into the lipid bilayer. Calcium is bound with an association constant of 151 M-1, while a phospholipid titration in the presence of calcium gave an association constant of 5 x 10(3) M-1 for egg phosphatidylcholine. Magnesium and cadmium cause very slight fluorescence enhancements, but a more significant effect is caused by the trivalent lanthanide ions. Analysis of these data indicates that the calcium-selective site is on the peptide and that ion binding to the phospholipid headgroups has a secondary role. Comparison with the divalent cation dependent antibiotics bacitracin and amphomycin shows that LY146032 has a quite different activity and that a calcium-dependent membrane interaction could account for results obtained in vivo.