Commercial in vitro diagnostic devices based on nucleic acid amplification systems such as polymerase chain reaction (PCR) are currently being submitted to the agency for evaluation. Although these DNA/RNA amplification systems are rapid and highly sensitive, technical difficulties can occur and lead to false results and misdiagnoses. Several substances (e.g., hematin, phenol) can inhibit DNA polymerases used in PCR diagnostics and cause false negative results. There are concerns that other substances could also inhibit these enzymes. We have optimized a PCR system for detection of DNA isolated from an attenuated strain (H37Ra) of Mtb and are using this system to screen various medications for their ability to inhibit PCR detection of Mtb. Thus far we have screened about 60 substances as possible PCR inhibitors: drugs used in the treatment of tuberculosis, other commonly prescribed drugs, and several over-the-counter medications. Most of the compounds examined did not inhibit PCR, a few were mild inhibitors of PCR, and one (containing ferrous sulfate) was found to be as potent as hematin, a well-known PCR inhibitor. Our screening system, several representative experiments, a summary of the results, and planned drug metabolite studies will be described.