Ethylene oxide appears to be an ideal sterilant for many heat sensitive items, but there are a number of disadvantages which require caution in its usage. Ethylene oxide has some deleterious effects on certain plastics and rubber. In particular, it acts as a solvent on such acrylic plastics as Lucite and Plexiglas , and it will attack the plasticizers found in many flexible medical-grade tubing formulations. Perhaps more significant is the fact that this sterilant can produce some serious toxic manifestations when medical plastics are not properly degassed. This paper reviews some of the toxicities already reported in the medical literature, and also presents new data on the hemolysis liability of many gas sterilized pharmaceutical plastics. Both human blood hemolysis studies and cell culture data on eighteen different polymers suggest that plasticized vinyl plastics have the highest toxicogenic potential. Experimental data presented on the solubility and partial pressure of ethylene oxide in a homologous series of phthalic acid esters indicate that the structure and concentration of plasticizers play a major role in the selection of the most effective degassing procedures to be used following ethylene oxide sterilization.