2 Abbreviations: BLM, bilayer lipid membrane.tion is the absence of tryptophan and the presence of six Cys residues.The second group of cytolysins, which is the most thoroughly studied, are the highly basic polypeptides ( I 9 and higher) with the molecular masses of ca. 20 kDa. They are isolated from various species of the Actiniidae and Stichodactylidae families, which mainly inhabit tropical regions of the ocean [1][2][3][4][5]14]. They were shown to exhibit a powerful lytic action on the sphingomyelin-containing membranes of eukaryotes by forming in them cation-selective ion channels (pores), due to which they were named actinoporins [1,15,16]. The absence of cysteine is characteristic of their amino acid composition. The amino acid sequences of eight actinoporins have been elucidated up to date. A steadily increasing interest of researchers to studying the structure and function of recombinant forms of actinoporins prepared by the methods of genetic engineering is now noteworthy [3].The third group of cytolysins comprises the lethal cytolytic phospholipases Ä 2 from Stichodactyla helianthus [17] and Aiptasia pallida [18] with M 30-40 kDa and similar in physicochemical properties cytolysins devoid any enzymatic activity [3].The only representative of the fourth group of cytolysins is metridiolysin, isolated from the sea anemone Abstract -Two cytolytic toxins (cytolysins Or-A and Or-G) were isolated from the Sea of Japan anemone Oulactis orientalis and characterized. Their purification scheme involved a hydrophobic chromatography on Polychrom-1, a gel filtration on Akrilex P-4, a cation-exchange chromatography on CM-32 cellulose, and a reverse-phase HPLC on a Nucleosil C 18 column. The molecular masses of Or-A and Or-G were determined by SDS-PAGE in 14% PAG to be ca. 18 kDa. The absence of Cys residues and a high content of basic amino acid residues are characteristic of their amino acid compositions. The hemolytic activities of Or-A and Or-G were found to be 295.86 and 322.58 HU/mg, respectively; these are by three orders of magnitude lower than those of sphingomyelin-inhibitable cytolysins from the tropic sea anemones. The amino acid sequences of the N -terminal fragments of Or-A and Or-G were determined to be ATFRVLAK and GAIIAGAA, respectively. Action of the cytolysins on the erythrocyte membrane is inhibited by exogenous sphingomyelin. They form ion channels in bilayer lipid membranes with the conductivity of 16, 32, and 40 pSm in 0.1 M NaCl and 168, 240, and 320 pSm in 1 M NaCl at pH 7.2. Therefore, they were attributed to the group of actinoporins.