Questions have been raised about the use of chemoprophylaxis in situations not fully addressed in the 1996 recommendations, including when not to offer PEP, what to do when the source of exposure or the HIV status of the source person is unknown, how to approach PEP in HCW's who are or may be pregnant, and considerations for PEP regimens when the source person's virus in known or suspected to be resistant to one or more of the antiretroviral agents recommended for PEP.