An enterotoxin synthesized during exponential growth by Bacillus cereus produces fluid accumulation in rabbit ileal loops, alters vascular permeability in the skin of rabbits, and kills mice when injected intravenously. All activities are eluted simultaneously from a Sephadex G-75 column and are distinct from the hemolysin and egg yolk turbidity factor of B. cereus. The enterotoxin is a true exotoxin. It interacts with intestinal receptor sites in a highly transient manner in the ileal loop system. Rabbit immune serum produced against the culture fluids from one strain of B. cereus neutralized the three biological activities in all other strains tested except strain B-6-ac for which none of the activities were neutralized. Enterotoxin proved to be unstable under a wide variety of conditions; ionic strength was especially critical. Enterotoxin was most stable in a pH range of 5.0 to 10.0, but lost activity rapidly outside this range. Alkylation provided some protection of enterotoxin activity in crude preparations but failed to protect activity during purification procedures. It did not appear to affect critically the enterotoxin molecule itself, since elution profiles on Sephadex G-75 chromatography were unchanged after alkylation.