Amplification and overexpression of the erbB-2 oncogene is an unfavourable prognostic marker in human breast cancer and occurs in approximately 25% of breast carcinomas. We used erbB-2 antisense oligonucleotides to inhibit the proliferation of human breast cancer cell lines. erbB-2 antisense oligonucleotides (20 microM) inhibited the growth and DNA synthesis of breast cancer cell lines with an amplified erbB-2 gene by up to 60%. Control complementary sense oligonucleotides did not inhibit cellular proliferation at the same concentration but showed inhibitory effects at higher concentrations. There was no specific effect of erbB-2 antisense oligonucleotides on breast cancer cell lines that had no amplification of erbB-2. erbB-2 antisense oligonucleotides reduced erbB-2 protein levels, measured by immunohistochemistry, in a dose-dependent manner. erbB-2 sense oligonucleotides did not decrease the levels of erbB-2 protein. These data indicate that erbB-2 antisense oligonucleotides induce a specific inhibition of erbB-2 protein expression and that erbB-2 gene overexpression is important for the proliferation of the breast cancer cells that have been selected for erbB-2 amplification.