We describe a synergistic effect of combinations of foscarnet and 3'-azido-3'-deoxythymidine against human immunodeficiency virus type 1 multiplication in cell culture, an additive effect of foscarnet and 3'-azido-3'-deoxythymidine triphosphate against human immunodeficiency virus type 1 reverse transcriptase, and a low toxicity in cell culture of combinations of the two drugs.The mechanisms of inhibition of human immunodeficiency virus (HIV) replication by 3'-azido-3'-deoxythymidine (AZT) and foscarnet in vitro have been well studied. AZT, which is a thymidine analog, is phosphorylated by cellular enzymes and, as a 5'-triphosphate, inhibits HIV type 1 (HIV-1) reverse transcriptase (RT) and terminates viral DNA chain elongation (8,15,22,24). Foscarnet interacts with HIV-1 RT at a site where pyrophosphate is split off during polymerization of nucleoside triphosphates (18,19,25). The activity of AZT TP is competitive with dTTP (24), whereas foscarnet is a noncompetitive inhibitor (25).The possibility of combining foscarnet and AZT against HIV infections and the possibility of using foscarnet against cytomegalovirus retinitis (20; S. L. Walmsley, E. Chew, S. E. Read, H. Vellend, I. Salib, A. Rachlis, and M. M. Fanning, J. Infect. Dis., in press) in patients with acquired immunodeficiency syndrome treated with AZT (6, 7) called for an evaluation of the effects of combining these two compounds. The combinations of AZT or foscarnet plus alpha interferon (12, 13) have shown synergistic effects. AZT plus ribavirin, on the other hand, has shown antagonistic effects (1, 23).The HIV-1 RT activity was assayed from lysed virus particles isolated from human T-lymphotropic virus type IIIB-infected U937 clone 2 cells (25). Foscarnet (Astra Alab AB, Sodertalje, Sweden), AZT (99% pure; Sigma Chemical Co., St. Louis, Mo.), and AZT TP (J. Chattopadhyaya, Biomedical Center, Uppsala, Sweden) were used. The viability of H9 cells (a human CD4+ lymphoid cell line) in cytotoxicity studies with antiviral compounds was assayed by counting the total number of cells in a hemacytometer and by trypan blue exclusion, both after 6 days of incubation. The mean of two experiments differing less than 15% is given. Cell inhibitory concentrations inhibiting culture growth to 50% of the control cell viability (CIC50) were calculated. The 50% inhibitory concentrations (IC50) of compounds were determined on HIV-1 replication. Uninfected H9 cells (105 cells per 0.5 ml) were seeded in 24-well microplates (Costar, Cambridge, Mass.) with 0.5 ml of medium with AZT or foscarnet or both in all possible combinations. Immediately after the cells and drugs were mixed, 1 ml of HIV-1 (supernatant fluid of persistently human T-lymphotropic virus type IIB-infected H9 cells) (10, 16) was added in concentrations giving 50 to 60% and 30 to * Corresponding author. 40% infected cells. After a 6-day incubation, HIV-1 antigen content was measured in the cells by immunofluorescence (9) and in the supernatants by enzyme-linked immunosorbent assay (V.-A. Sundqvist, J. Albert, ...