HIV-specific cytotoxic T lymphocytes (CTL) are believed to play a major role in controlling virus levels through the asymptomatic period of HIV infection. For the rational design of an HIV vaccine, we need to know whether protective immunity can ever develop following HIV exposure in people who remain uninfected. We have detected HIV-specific CTL in 5/6 repeatedly exposed, persistently seronegative female sex-workers in The Gambia. Their CTL, repeatedly detected over two years, recognise epit...