DNA of human papillomavirus type 18 is present in several human cancer cell lines that were derived from oral or cervical tumors, and it is known that several features of the transformed phenotype can be inhibited by expression of antisense RNA to human papillomavirus (HPV). The present study was performed to find out whether antisense oligonucleotides were also inhibitory. Synthetic oligonucleotides were made that were complementary to regions of the start codons of the E6 and E7 genes of HPV-18. These were added to cultures of the oral cancer cell line 1483 and the cervical cancer cell line C4-1, each of which contain DNA of HPV-18. As controls we used the oral cancer cell line 183 and the monkey kidney cell line Vero, which do not contain HPV. Anti-E6 and anti-E7 oligonucleotides, in concentrations between 1 and 5 microM, significantly inhibited the growth of the 1483 and C4-1 cells, but not the 183 or Vero cell lines. Treatment of the 1483 cells with a combination of 2.5 microM of each of the antisense oligonucleotides was a more effective inhibitor than 5 microM of either one used alone. Antisense oligonucleotides had no effect on the ability of 1483 cells to form foci in soft agar, nor on their plating efficiency or serum requirements. Microscopic examination of 1483 cells showed that the antisense E7 oligonucleotide produced cell-rounding, detachment from the surface of the culture flask, and cell death, while the antisense E6 oligonucleotide had none of these effects. Random-sequence oligonucleotides had no effects of any type on any cells that were growing in culture. However, if random-sequence oligonucleotides were added to cells at the time they were passed to a new culture vessel, they produced severe nonspecific toxic effects. These results show that the use of synthetic oligonucleotides is an effective way of producing antisense-mediated changes in the behavior of human cancer cells that contain DNA of HPV-18.