Chemical modification studies have been carried out on the sea anemone polypeptide anthopleurin-A in order to clarify the role of Arg-14 in its cardiac stimulatory activity. Reaction with 1,2-cyclohexanedione at 37 °C produced a range of protein products, including some with amino group modifications. These side-reactions were eliminated by prior citraconylation of the amino groups, which, following reaction with cyclohexanedione, could be reversed under conditions which preserved the cyclohexanedione adduct. Citraconylation of the three amino groups, one from the N-terminus and two from Lys-37 and Lys-48, destroyed the cardiac stimulatory activity of the molecule, but this was fully recoverable upon reversal of this reaction. It appears that one or more of the amino groups is essential for activity. Anthopleurin-A contains only one arginine residue, and this was confirmed as the site of modification by cyclohexanedione by showing that the product was refractory to proteolysis by trypsin, which normally cleaves the molecule at this residue. The positive inotropic activity of the cyclohexanedione adduct on isolated guinea-pig atria was identical to that of unmodified anthopleurin-A, indicating that the side-chain of Arg-14 is not required for cardiotonic activity.