The presence of free infectious HTV in the plasma frac-tion of blood (plasma viremia) has been well estab-lished since the initial descriptions of cases involving hemophiliacs|| 1–3]. Since then, a number of studies using in vitro culture systems and other methods have confirmed the presence of free infectious virus in plasma. In plasma, HIV is exposed to, and interacts with, a number of components. By far the most intensively studied of these interactions is between HIV and antibody. This review will focus on non-antibody factors in plasma with which cell-free HIV has been shown to interact. The basic characteristics of plasma virus are also discussed.