In t(14;18) lymphomas, bcl-2 gene is activated by the juxtaposition of immunoglobulin (Ig) gene. The fused bcl-2-Ig gene generates chimeric mRNAs which consist of bcl-2 at 5' side and Ig at 3' side. Chimeric mRNA does not disrupt the bcl-2 coding frame of 239 amino acid polypeptide. Activated bcl-2 gene introduced in normal B lymphoblastoid cells (LCL) demonstrated an increased cloning efficiency in soft agar but failed to confer tumorigenicity to LCLs as a single agent. bcl-2 gene rearrangement in Japaneses B cell lymphoma was studied and found that 10 out of 32 cases of follicular lymphoma (31%) and 5 out of 56 cases of diffuse lymphoma (9%) were rearranged, suggesting less frequency of B cell lymphoma, particularly follicular lymphoma in Japan is partly due to less bcl-2 involvement than American cases. Three cases out of 15 cases with bcl-2 rearrangement demonstrated a unique pattern of rearrangement. Two cases of the three were analysed and found that both cases were translocated at the later step than DH-JH joining of Ig rearrangement. Thus, bcl-2 translocation in Japanese B cell lymphomas might occur at the later stage of B cell development, when compared with that in American cases. Less involvement of bcl-2 in Japanese B cell lymphoma may be explained by low susceptibility to bcl-2 rearrangement at the step of DH-JH recombination.