The efficiency and duration of transmissibilty of HIV seems to be highly variable and dependent on a number of factors related to both the donor and the recipient as well as characteristics intrinsic to the virus itself. Some of the factors likely to be important in sexual transmission of HIV include stage of disease, antiretroviral therapy, and concomitant systemic or mucosal infections, including sexually transmitted diseases. This paper describes recent work from our group in each of these areas.