Human papillomavirus (HPV) infection is believed to play a central role in cervical carcinogenesis. Specifically, two viral oncoproteins, E6 and E7, possess transforming ability and have been shown to interact with the cellular tumor suppressors p53 and p105, the retinoblastoma (Rb) gene product. To test the hypothesis that E6 and E7 play an active role in the maintenance of the malignant phenotype and may be ideal targets for antigene therapy, we tested the antiproliferative effects of phosphorothioate oligodeoxynucleotides (oligos) targeting HPV-16 E6 and E7 in cervical cancer cell lines and primary tumor explants.The ATP cell viability assay was used to measure growth effects of 27-mer antisense oligos targeting the ATG translational start region of HPV-16 E6 and E7 sequences in HPV-16-positive cell lines SiHa and CaSki and four advanced, primary cervical tumor explants. A random oligo sequence, an HPV-18-positive and HPV-negative cell line, one histologically confirmed endometrial and two ovarian tumors were used as negative controls. HPV type was confirmed by hybrid capture techniques. Cell lines and sterile (staging laparotomy) tumor cells were plated at 5000 cells/0.1 ml and 100,000 cells/0.5 ml in 96-well plates or soft agar, respectively, and incubated at 37°C with a single treatment of oligos at 0–16 μ M . E6/E7 combinations at a fixed ratio of 1:1 were used at 0–8 μ M for each oligo. Cellular ATP was measured by luciferin/luciferase fluorescence on Day 6. HPV-16 E6 and E7 oligos showed antiproliferative effects in all HPV-16-positive cell lines and primary tumor explants (IC 50 s 6.9–9.5 μ M for cell lines, 9.1–12.1 μ M primary cervical tumors), while the HPV-negative C33-A cell line and HPV-18-positive cell line HeLa were relatively insensitive to the HPV-16 oligos (IC 50 s > 30 μ M extrapolated). The endometrial and two ovarian primary tumors were also insensitive to the HPV E6 and E7 oligos (IC 50 s > 25 μ M extrapolated). Random oligos had little effect on cell growth at concentrations up to 16 μ M (< 25% inhibition), except in CaSki (@50% inhibition at 16 μ M ). Combinations of E6 and E7 demonstrated mixed synergistic and antagonistic effects as determined by combination indices (CI) derived from median effect parameters. In the HPV-16-positive primary cervical tumors and the cell line SiHa, E6/E7 combinations were synergistic at low doses (<25% growth inhibitory dose range) and antagonistic at doses above this. For the HPV-16-positive cell line CaSki, however, E6/E7 combinations were antagonistic at all dose ranges. Phosphorothioate oligos directed against the viral oncogenes E6 and E7 were shown to have antiproliferative effects specific to HPV-containing cancer cells. These specific antiproliferative effects suggest that HPV-16 E6 and E7 sequences play an active role in the malignant growth properties of cervical cancer cells and may be ideal targets for antigene therapy.