Southern blot hybridization was used to detect the rearrangement and amplification of five proto-oncogenes (bcl-2, bcl-1, c-myc, c-myb and c-Ha-ras) and one tumor suppressor gene (RB-1) in 55 Japanese patients with non-Hodgkin's lymphoma; 16 with T-cell lymphomas and 39 with B-cell lymphomas (7 follicular and 32 diffuse lymphomas). Genetic abnormalities of the proto-oncogenes were detected in 7 of the 55 (13%). Genetic abnormalities of bcl-2 plus other genes were detected in 5 of 7 cases of follicular lymphoma (71%), rearrangements of bcl-2 and c-myc, rearrangement of bcl-2 and amplification of c-myb. Genetic abnormalities were observed in only three cases of diffuse lymphoma. In each of 3 cases of B-cell lymphoma, one of the genes, blc-2 mbr, bcl-2 mcr and c-myc, was rearranged respectively. The incidence of genetic abnormalities in diffuse lymphomas (6.3%) was lower than that in follicular lymphomas. None of diffuse lymphomas had double oncogene abnormality. No abnormalities were found in RB-1, bcl-1, and Ha-ras. These findings suggest that follicular lymphomas are associated with some abnormalities of oncogenes not restricted to bcl-2 that facilitate growth which may be associated with their clinical features.