Intestinal protozoa like Entamoeba histolytica and Giardia lamblia have been proposed as vectors or cofactors in the development of AIDS. To determine whether these protozoa could transmit HIV, laboratory strains of protozoa were cocultured with cells chronically infected by a highly replicative strain of HIV-1. Entamoeba histolytica, but not Giardia lamblia, took up virus. Immunologically detectable HIV-1 was present in the amebae up to 48 h after exposure to infected cells, but this virus could not be transferred to uninfected human cells. Amebae isolated directly from two HIV-infected individuals were also found to be positive for HIV-1. After lysis of these protozoa and coculture with uninfected peripheral blood mononuclear cells, no transfer of virus to the human cells was observed.