Although the density of the HIV envelope protein (gp160) can vary on cells and virions, there has been little investigation into how the levels affect HIV infection and cell-to-cell fusion. To address this several cell lines stably producing infectious, replication defective virions of the T-cell line adapted HIV isolate, HXB2, were made. The cell lines varied from one another in gp160 surface expression over a five-fold range, while expressing similar levels of the Gag polyprotein. As a consequence, the cell lines collectively produced virus that varied in gp160 content over a six-fold range. With the cell lines it was observed that increasing gp160 surface expression increases cell-to-cell fusion efficiency and syncytium size. With the virus preparations it was found that gp160 levels had a biphasic affect on infection efficiency. Virus preparations with gp160 levels of 10-30 molecules per virion showed that increased gp160 content correlated with increased infection efficiency. However, at gp160 levels above 50 molecules per virion there was a decrease in infection efficiency associated with increasing gp160 per virion content. Therefore, it appears that increasing gp160 surface density exerts a qualitatively different affect on infection and cell-to-cell fusion. Based upon functional analysis, several cell lines producing the gp160 molecule from the primary isolate, JR-FL were also made. These cell lines were less characterized, and yet have the potential to be useful reagents in HIV research as well.