The human colon epithelial cell line HT29 can be infected by selected strains of the human immunodeficiency virus (HIV) [9]. In the present study, it is shown that tumor necrosis factor-alpha (TNF-alpha) is a potent stimulator of HIV replication in chronically infected differentiated HT29 cells, but not in undifferentiated cells. The polarity of HIV production upon TNF-alpha stimulation has been studied in polarized monolayers of differentiated HT29 cells grown on porous-bottomed dishes. It is shown that the cytokine induced a dramatic increase of HIV production through the two opposite sides of the monolayer, i.e. the apical and basolateral plasma membrane domains. The effect of TNF-alpha was mainly localized at the level of viral mRNA synthesis as demonstrated by in situ hybridization. These data support the concept that cytokines released as a result of intestinal inflammatory responses could promote HIV replication and contribute to the gastrointestinal disease in HIV-infected patients.