We are establishing, modifying, and developing at CBER sensitive assays for the detection of retroviruses. With the recent publication of three extremely sensitive assays to measure reverse transcriptase (RT) activity and the finding that certain licensed vaccines contain RT activity, it became necessary that CBER set up and investigate these assays biochemically in order to ascertain what precisely they were measuring. Because RT is usually associated with a retroviral particle, it was possible that those vaccines in which RT activity was found were contaminated with low amounts of a retrovirus whose presence was only revealed by the new and more sensitive assays. In fact, these sensitive RT assays do detect minor activities of cellular enzymes, and part of our program at CBER is to develop assays capable of discriminating between the two. This will be important in order to be able interpret any positive signals found in biological products destined for human and veterinary use. Our work involves using applying and developing assays to measure the presence of very low levels of retrovirus contamination in vaccines and cell substrates. The use of such methods is critical to the mission of the FDA in ensuring the safety of the products we regulate. Because of the ever-present possibility that new viruses that are pathogenic to humans will emerge, it should be obvious to all that CBER must be able to respond quickly to contingencies as they arise. To do this, it is necessary to have in-house expertise capable of applying, modifying, and developing new assays.