Two cytolysins, designated as magnificalysins I and II, were purified from a tropical sea anemone, Heteractis magnifica (formerly Radianthus ritteri). The purification steps involved Sephadex G-50 and CM-Sepharose chromatography followed by Mono S and Phenyl-Superose Fast Protein Liquid Chromatography. The relative mol. wt of magnificalysins I and II, determined by SDS-PAGE, was approximately 19,000, while their isoelectric points, determined by isoelectric focusing in immobilized pH gradients, were 9.4 and 10.0, respectively. Those toxins were found to have haemolytic and lethal activities. The haemolytic activities of magnificalysins I and II were 3.6 x 10(4) HU/mg and 3.3 x 10(4) HU/mg, while their LD50 (i.v., mice) values were approximately 0.14 microgram/g and 0.32 microgram/g, respectively. The amino acid composition and N-terminal sequences of magnificalysins I and II were also obtained. They do not possess any cysteine or cystine residue, but are rich in basic and hydrophobic amino acids. The N-terminal amino acid sequences of magnificalysins I and II are ALAGTIIAGASLTFKILDEV and SAALAGTIIDGASLGFDILNKV, respectively. These are highly homologous to cytolysins from other sea anemones, particularly cytolysin III from Stichodactyla helianthus, a Caribbean anemone.