Intravenous injections of 7,12-dimethylbenz[a]anthracene (DMBA) induce erythroblastic leukemia (erythroleukemia) with #2 trisomy and Long #2 in Long-Evans rats. Recently, a consistent type of mutation, A to T transversion in codon 61 of N-ras gene, was found in all of 6 cultured leukemia cell lines and 13 primary leukemias induced by DMBA using polymerase chain reaction (PCR) and direct sequencing. On the contrary, no mutation was observed in Ha- and Ki-ras genes in these leukemias. The consistent occurrence of the above N-ras mutation in DMBA-induced leukemias indicates that N-ras gene plays an important role in DMBA-leukemogenesis. Mutations in ras genes generally takes place during the initiation stage of carcinogenesis because they often appear in the premalignant stage of tumors. In order to detect the N-ras mutation in an early stage of leukemogenesis, we designed the mutant-allele-specific amplification (MASA) method to detect the mutation in bone marrow (BM) cells of DMBA-treated rats. The MASA method was sensitive enough to detect one mutant cell mixed in 10(6) normal cells. Using this method, the N-ras mutation was found in BM cells 2 days after single DMBA injection and thereafter throughout the preleukemic stage. These results suggest that the N-ras mutation is an earliest event in DMBA-induced leukemogenesis.