An intracellular toxic protein of Pseudomonas aeruginosa was purifi ed by ion-exchange chromatography on DEAE-cellulose. The purifi ed protein was homogeneous as determined by polyacrylamide disc gel electrophoresis; its molecular mass was determined by gel fi ltration on Sephadex G-150® and was estimated to be about 62 000 kDa. The purifi ed intracellular toxin appeared to be a dimer and the molecular weights of the subunits were estimated to be 31 000 kDa and 30 500 kDa. The purifi ed protein showed signifi cant toxicity against brine shrimp nauplii and its LC50 value was calculated to be 25 μg/ml. The toxic protein was glycoprotein in nature with a neutral sugar content of 4%. It agglutinated red blood cells from albino rats and rabbits and the agglutination was strongly inhibited in the presence of galactose. This intracellular toxin may be an important virulence factor of P. aeruginosa and might be involved in the pathogenesis in human infections.