Although chromosome aberrations in T [thymus-derived] lymphocytes and bone marrow cells were reported in atomic bomb survivors, the presence of chromosome abnormalities was not demonstrated in B [bone marrow-derived] lymphocytes because of the technical difficulties involved in B-lymphocyte separation. A method for detecting chromosome aberrations in B lymphocytes was established by stimulation of B lymphocytes with Epstein-Barr virus (EBV) instead of separation of B lymphocytes by rosette formation. The EBV-stimulated lymphocytes were isolated as single colonies in soft agar and transferred to liquid culture for further cell growth. The EBV-stimulated B lymphocytes of 2 heavily exposed survivors showed 50% and 12.5% chromosome abnormalities 30 yr after exposure to the effects of the atomic bomb. The former patient seemed to have a karyotypically abnormal clone of B lymphocytes in vivo. The method used and the evidence of chromosome aberrations in B lymphocytes for long periods after radiation exposure will be useful and important in elucidating the malignant processes of acute lymphocytic leukemia, B-cell lymphoma, and multiple myeloma among high-risk groups having a history of accidental or therapeutic exposure to radiation or radiomimetic drugs.