Both the rearrangement and the expression of the bcl-2 gene in Japanese hematopoietic tumor cells were studied by Southern and Northern blot hybridizations. The expression of the bcl-2 gene was studied in seven cultured cell lines and twenty clinical samples, including eleven non-lymphoid tumors and nine lymphoid tumors. All lymphoid tumors except one sample from a patient with ALL expressed the bcl-2 gene. The bcl-2 gene was strongly expressed in B cell tumors. This gene was also expressed in T-ALL samples studied, although the expression was not as marked as in the B cell tumors. Non-lymphoid tumors did not express bcl-2 gene. bcl-2 gene rearrangement was studied in five cultured cell lines and six clinical samples including four follicular lymphomas and two T-ALLs. No abnormal bcl-2 gene configurations were found in any of the clinical samples. Among the cultured cell lines, the BALL-1 line showed two-fold amplification. The frequency of bcl-2 rearrangement in Japanese follicular lymphomas is reported to be lower than that seen in the American follicular lymphomas. Nevertheless, the increased expression of the bcl-2 gene seen in the Japanese B cell tumors studied supports the contention that the bcl-2 gene plays an important role in the pathogenesis of B cell tumors.