A common approach to sex work regulation is “legalization and regulation,” which aims to reduce infectious disease by providing government certification of female sex workers’ health. In markets where suppliers possess important, unobservable information, theory predicts that credible certification can improve welfare. However, in a sample of uncertified female sex workers, take-up of a randomly offered incentive to obtain certification was only 7%. The results suggest that there is no price premium for certification, and that internalized stigma deters women from seeking certification. Complementary services for uncertified sex workers may be needed to achieve sexually transmitted infection control.