SummaryStonefish venom has been fractionated on carboxymethyl cellulose and Sephadex columns; of the fractions obtained only one was toxic to rats. The active fraction contained a relatively high proportion of aromatic amino acids. Whole venom was examined for enzyme activity and enzyme inhibition; the only significant activity found was that of hyaluronidase. The venom also caused a marked increase in capillary permeability. Ultracentrifuge measurements suggest that the toxic material has a molecular weight of the order of 150,000.