We investigated mutations of N-RAS and K-RAS by using polymerase chain reaction (PCR)-oligonucleotide hybridization techniques in 40 cases of Chinese leukaemia patients and 17 presently healthy members of a family with high incidence of acute myeloid leukaemia. The results showed only two patients carried the mutation in codon 12 of N-RAS. Strikingly, however, in both cases the malignancies involved lymphoid lineage. There was no hereditary RAS mutation in the members of the remarkable family.