To examine whether purified tat retained biological activity, a modified scrape loading technique was used to introduce the protein into cell monolayers. In this procedure, either a monolayer culture of HeLa cells transfected 40 hr prior with plasmid pU3R-III or a monolayer culture of HeLa cell line that surface of the dish with a rubber policeman in the presence of the purified tat. Following the scraping procedure, cells were centrifuged, replated in fresh media, and CAT assays were performed 4 hr after addition of protein. As graphically elevated in cells that received that tat.