HIV-1 is routinely isolated by cocultivation of patient PBMC with mitogen-stimulated HIV-uninfected donor PBMC (see Chapter 1). In this culture system, HIV-1 primarily replicates in CD4⁺ T-lymphocytes, and such viruses are termed clinical or primary isolates. As early as 1986, the in vitro replicative capacity and cell tropism of primary HIV-1 isolates were shown to be important in the pathophysiology of HIV-1 infection (1). High replication capacity in PBMC and virus growth and syncytium formation in neoplastic T-cell lines were found to correlate with severity of HIV-1-disease (2–5). Compared to syncytium-inducing (SI) isolates, nonsyncytium-inducing (NSI) strains did not replicate in neoplastic T-cell lines and showed preferential replication in cells of the monocyte-macrophage lineage (6,7). Thus, NSI viruses have often been termed macrophage tropic, whereas SI strains are termed T-cell line tropic.