VSV-G pseudotyped HIV-1 is more stable over time in BeWo wild type cells (A) and in BeWo-CD4+ cells (B) compared to BaL, HXB2 and envelope-deficient HIV-1 virions. Following viral exposure, the viral suspension was removed, and replaced with fresh medium; cell samples were collected at different time points. The amount of intracellular p24 antigen present in cells lysed immediately after viral exposure (time 0) was considered to be 100%, and all subsequent time points were normalised to time 0. The results are the mean of at least three independent experiments. Error bars indicate SEM.