Human immunodeficiency virus type-1 is highly variable and utilizes two independent pathways, one via the CCR5 coreceptor and the other via CXCR4 to enter CD4+ cells. Both virus types can simultaneously be found in HIV-1-infected patients and the shift to a majority of CXCR4-tropic viruses is an indication for the development of AIDS-related symptoms. It is thought that each virus type will infect cells independently using its specific coreceptor pathways. In this short communication we demonstrate that CCR5-tropic HIV-1 is enhanced by the envelope from CXCR4-tropic viruses. This cross-enhancing effect was completely suppressed when the target cells were pretreated with hexadimethrine bromide, suggesting that carbohydrate structures on the cellular membrane play an important role for envelope-dependent enhancement of viral entry.