Mutation of the RAS oncogene was studied in ten patients with multiple myeloma, and the DNA from nude mouse tumors formed by cells obtained from tumorigenecity assays (in vivo selection assays) in these patients was analyzed by PCR and oligonucleotide hybridization. Mutations of the N-RAS oncogene were identified in two of three patients investigated by in vivo selection assay and in five of ten patients investigated by PCR analysis of DNA from myeloma cells. In the two former patients, mutation of the N-RAS oncogene was observed at the 61st codon. Of the five N-RAS mutant-positive patients investigated by the PCR analysis, one had a mutation at codon 12, two had mutations at codon 13, and two had mutations at codon 61. None of the patients had mutations of the K-RAS oncogene. These results suggest that the frequency of RAS gene mutation in multiple myeloma is higher than in other lymphoid malignancies such as acute lymphocytic leukemia, chronic lymphocytic leukemia, and malignant lymphoma. As the mutation was observed only at the N-RAS oncogene level, it is speculated that N-RAS oncogene activation might play an important role in the progression of multiple myeloma.