The many benefits of breastfeeding are well documented. However, because of the risk of mother-to-child transmission (MTCT) of HIV from an HIV infected mother to her infant, there is considerable concern over the practice, especially in developing countries. The purpose of this study is to determine the safety and effectiveness of the anti-HIV drug nevirapine (NVP) in preventing MTCT of HIV in breastfeeding infants born to HIV infected women in South Africa, Tanzania, Uganda, and Zimbabwe.