Abstract: Structure for evaluating the efficacy of sterilization apparatus using either steam or ethylene oxide as sterilant is disclosed. The structure includes an insert of porous material having a cavity formed therethrough for retaining a biological or chemical indicator. The porous material has homogeneous porosity throughout. The insert of porous material and indicator are enclosed within an outer covering which is relatively impervious to penetration of gases and which provides restricted pathways to air and sterilant. The outer covering is provided with removable portions which are removed when the structure is used to evaluate ethylene oxide sterilization. Removal of the portions enlarges the existing pathways for ingress and egress of gases which may be appropriate in certain efficacy testing.