Two new serine proteinase inhibitors (RmIn I and RmIn II) from the tropical sea anemone Radianthus macrodactylus have been isolated and characterized. The purification procedure includes polychrome-1 hydrophobic chromatography, Superdex™ Peptide 10/30 FPLC, and Nucleosil C18 reverse-phase HPLC. The molecular masses of RmIn I, RmIn II, and the complexes RmIn II/trypsin and RmIn I,II/α-chymotrypsin have been determined. The Ki values of RmIn I and RmIn II for trypsin and α-chymotrypsin have been determined. The polypeptides RmIn I and RmIn II are shown to be nontoxic and to exhibit antihistamine activity. The N-terminal amino acid sequences of RmIn I (GICSEPIVVGPCKAG-) and RmIn II (GSTCLEPKVVGPCKA-) have been determined. A high homology of the amino acid sequences is demonstrated for the proteinase inhibitors produced by such evolutionarily distant species as coelenterates, reptiles, and mammals.