We purified a new cytolysin (HMgIII) from the sea anemone, Heteractis magnifica. HMgIII, which has a molecular mass of approximately 19 kDa, functions as both a cytolysin and a hemolysin. The full-length HMg III cDNA was obtained by reverse transcriptase-polymerase chain reaction, using primers designed from its N-terminal amino acid sequence and an internal conserved region of two other sea anemone cytolysins: equinatoxin II (EqT II) and cytolysin III. The cDNA contained an open reading frame of 633 bp, which encodes a protein of 211 amino acids. The nascent HMg III protein contained a prepropeptide of 34 amino acids, which includes a signal peptide of 19 amino acids. The mature HMg III has a predicted molecular mass of 19 kDa and a pI of 9.1, and shares 91%, 89%, 65% and 63% amino acid sequence similarity with cytolysin III, cytolysin ST I, tenebrosin-C and equinatoxin (EqT II), respectively. The predicted secondary structure of the mature HMg III comprises 16% alpha-helix, 23% extended strand and 60% random coils. The characteristic amphiphilic alpha-helix of cytolysins is located at the N-terminus of the processed HMg III. Recombinant HMg III (rHMg III) was expressed in Escherichia coli as a fusion protein containing a 6xHisTag at the N-terminus. The hemolytic and cytotoxic activities of the purified rHMg III were comparable to those of the native HMg III. The hemolytic activities of both proteins were similarly potentiated with 8-anilino-1-naphthalenesulfonate (ANS). Increasing the length of the peptide tag on the N-terminal of rHMg III correlated with decreasing hemolytic activity, thus confirming the importance of the N-terminal amphiphilic alpha-helix for its cytolytic activity.