Two hemolysins, Sticholysin I (St I) and Sticholysin II (St II) were purified from the sea anemone Stichodactyla helianthus combining gel filtration and ion exchange chromatography. The amino acid composition of both cytolysins was determined revealing a high proportion of glycine, lysine, tyrosine and non-polar amino acids (alanine, leucine and valine). Cysteine was not found in either polypeptide. Molecular masses of St I and St II were 19401 and 19290 Da, respectively. N-terminal sequence analysis of St I and St II showed a high homology between them suggesting they are isoforms of the same cytolysin. Compared with other sea anemone cytolysins, St I and St II contain a 22 amino acid insertion fragment also present in Eq T II/Tn C and probably in CaT I and Hm T and absent in C III, the major hemolysin previously reported in this anemone.