The severely injured cells of vertebrate material seem to release in inflammatory exudates a toxic material located or associated with the euglobulin fraction of exudates. This toxic substance, which, per se, offers a reasonable explanation for the basic pattern of injury in inflammation, has been termed "necrosin." What appears to resemble this state of affairs is likewise encountered in severely damaged tissues of some invertebrates. Crushed tissues of either Mya arenaria or the gills of Limulus polyphemus yield an extract which is toxic to otherwise normal M. arenaria. The toxicity seems to reside primarily in the protein fraction obtained by treating the extract with (NH4)2SO4 at 1/3 saturation. This chemical procedure in vertebrate material yields, after dialysis of the sulphate ions, what is termed the "euglobulin fraction." It is in association with the euglobulin fraction of inflammatory exudates that necrosin is recovered. The toxic euglobulin factor in the injured tissue of the invertebrates studied is thermolabile. Boiling tends to inactivate it in large part. This is a similar property possessed by necrosin. The studies suggest that the toxic factor in the severely injured tissues of the invertebrate forms studied is similar to, or identical with, necrosin recovered from the products of cellular injury in vertebrates. There may be an''ad-ditional thermostable toxic factor in the injured tissues of M. arenaria.