Natural-abundance 13C NMR spectroscopy at 15.04 MHz has been used to examine the effects of pH, calcium, and lanthanide ions on the polypeptide cardiac stimulant Anthopleurin-A in aqueous solution. The carboxyl resonance from the aspartic acid residue not observed in a previous study (R. S. Norton and T. R. Norton, 1979, J. Biol. Chem. 254, 10220-10226) has been identified and an apparent pKa of 3.4 obtained. More accurate estimates have been derived for the apparent pKa values of the two histidine residues. Binding of Ca2+ ions has been found by equilibrium dialysis and 13C NMR to be weak (Kd > 0.1 M). The interaction with lanthanide ions is slightly stronger, but binding occurs at the C terminus as well as at a site involving one or both of the aspartate carboxylate groups. These results suggest that possible Anthopleurin-A-induced calcium translocation in the myocardial cell is a secondary effect. The interaction of Anthopleurin-A with lipid monolayers has also been examined. Binding occurs to neutral and zwitterionic lipids, but is stronger with negatively charged lipids, particularly cardiolipin. This interaction is also influenced by the presence of Ca2+ ions. The implications of these results for the mechanism of action of this polypeptide are discussed.