Antisense oligodeoxynucleotides targeted to Ha-ras mRNA have been designed to discriminate between the codon 12-mutated oncogene and the normal proto-oncogene. An in vitro assay using two different sources of RNase H (rabbit reticulocyte lysates and nuclear extract from HeLa cells) was used to characterize oligonucleotide binding to normal and mutated Ha-ras mRNA. Short oligonucleotides (12- or 13mers) centered on the mutation had a very high discriminatory efficiency. Longer oligonucleotides (16mers) did not discriminate efficiently between the mutated and the normal mRNA. We have tested the efficacy of dodecanucleotides to induce RNase H cleavage of the full-length mRNA, moving the target sequence from the loop to the stem region which is formed in the vicinity of mutated codon 12. The most selective oligonucleotides were centered on the mutation which is located near the junction between the loop and stem regions even though they were less efficient at inducing RNase H cleavage than those targeted to the loop region. The 12mer antisense oligonucleotide with the highest discriminatory power was selected for cell culture studies. This oligonucleotide inhibited the proliferation of a human cell line which had been transformed with the mutated Ha-ras gene (HBL100ras1) but had no effect on the parental cell line which was transfected with the vector DNA (HBL 100neo) and expressed only the normal Ha-ras gene. Growth inhibition of HBL100ras1 cells was associated with specific ablation of targeted Ha-ras mRNA as shown by RT-PCR. These results show that 'in vitro' evaluation using an RNase H assay allowed us to select an antisense oligonucleotide which elicited a selectivity towards point-mutated Ha-ras mRNA when added at 10 microM concentration to the culture medium of cells expressing wild type and mutated Ha-ras mRNA.