PDA Technical Report No. 51 provides a comprehensive review of an area not adequately addressed in current guidance documents: recommended specifications for BIs to be used with sporicidal gas and vapor-phase decontamination cycles together with guidance regarding their manufacture, quality control, and use. The principles described in this report are based upon the manufacture and use of BIs prepared from spore suspensions; however, they can be equally applied to the preparation of BIs from other sources. The science and current understanding of biological inactivation mechanisms for sporicidal vapor-phase decontamination processes are included in this guide, along with details relating to BI specification and guidance concerning manufacturing methods. Since BIs are intended for use in the qualification of vapor-phase decontamination cycles used in CGMP aseptic processing and in critical bio-safety applications, recommendations for quality assurance and quality control in the manufacture and use of BIs are also included. Sporicidal decontamination processes include, but are not restricted to, hydrogen peroxide vapor, peracetic acid vapor, chlorine dioxide gas, and formaldehyde separately and possibly in combination with more than one agent. These recommendations are based on current practices using hydrogen peroxide vapor alone (or in combination with peracetic acid), but may be equally applied to other agents used in similar gaseous or vapor based processes. Examples based on the use of hydrogen peroxide are included. Ethylene oxide is not be discussed in this document, since it is used almost exclusively in the context of sterilizers used for processing items such as thermolabile medical devices.