Evidence has accumulated indicating that macrophages could play a role in the physiopathology of AIDS. We recently demonstrated that cultured human liver macrophages, the so-called Kupffer cells (KC), are permissive for HIV. Their infection in vivo would lead these cells to constitute a target for the virus and a reservoir as well. Since they occupy a strategic position within the liver sinusoid, their opportunity to interact with blood-borne virus or already infected T lymphocytes may be very high. In the present study, we investigated the possibility for KC to be infected via HIV-infected CEM cells, a lymphoid cell line. Therefore, we cocultured both cell types for various times before fixing them for electron microscopy. Syncytia appeared within 20 h of infection as well as a large amount of virus particles. HIV in the way of budding was also easily observed. This has to be compared to the direct infection of KC with free virus which needs, at least, about 10 days to give the same results.