Unusual forms of gas bacillus infection occurring in modern times are reviewed, with emphasis on the clinical and pathologic findings of the endogenous form of the disease. Two recently observed cases of endogenous gas bacillus infection were observed; one was diagnosed before death by blood culture. One case represented the darmbrand type of infection which was secondary to necrosis of the intestine in a patient who had acute lymphocytic leukemia. The portal of entry of the Clostridium welchii was the necrotic intestine. The portal of entry in the second case was the biliary tract, following biliary-tract obstruction and cholecystitis. Endogenous gas bacillus infection is a medical emergency requiring early recognition and intensive therapy consisting of debridement, penicillin, polyvalent antiserum, and blood transfusions.