How do firms decide to provide HIV/AIDS prevention services? In this CGD Working Paper, Visiting Fellow Vijaya Ramachandran analyzes data from 860 firms and 4,955 workers in Uganda, Tanzania, and Kenya. She finds that larger firms, and those with more highly skilled workers invest more in AIDS prevention. Firms in which more than 50 percent of workers are unionized are also more likely to do more prevention activity. Finally, these characteristics are significant in determining whether or not a firm carries out pre-employment checks of its workers. The results shed light on the likelihood of private intervention and the gaps that will require public sector assistance.