In a study of experimentally induced open tibial or femoral fractures in rats, after either closing or leaving the wounds open, the animals were given: no antibiotic, cephalothin (Keflin), or penicillin. The rats with wounds closed primarily and receiving no antibiotics had the highest mortality rate (11 of 25) from experimentally produced clostridial myonecrosis. The lowest overall mortality rate (5 of 99) was found in the penicillin-treated groups. The higher mortality rate in the femur fracture groups was probably because of the large muscle mass of the thigh. The importance of ideal anaerobic conditions for producing experimental clostridial myonecrosis is emphasized.