By means of a bacterial monitor, it was found that a 2 megarad dose of high velocity electrons will sterilize 0.353 inch of polyethylene. A new bacterial monitor was devised which serves as a direct measure of the sterilizing effect of irradiation on microorganisms. The bacterial monitor is a sealed polyethylene envelope containing polyethylene strips infected with spores of Bacillus thermoalimentophilus. The bacterial monitor may be used to establish the sterilizing capabilities of electron accelerators and may be used as a routine control check on sterilization by irradiation.