bcl-2 protein has been detected in surgical specimens and cultured permanent cell lines of non-Hodgkin's lymphomas and leukemias using enzyme immunohistochemistry and immunofluorescence with anti-bcl-2 monoclonal antibodies. Of 40 surgical specimens, bcl-2 protein was expressed in 50% of B-cell and 41% of T-cell lymphomas, both with and without the bcl-2 gene rearrangement. In investigations of 38 hematopoietic cell lines, bcl-2 protein was detected not only in lymphoid cell lines but also in myeloid cell lines. In situ hybridization and immunohistochemical analysis of reactive lymph nodes showed that lymphocytes in mantle zones and paracortical areas expressed bcl-2 protein consistent with the messenger RNA distribution and that germinal center cells showed abundant bcl-2 transcript, despite the absence of detectable bcl-2 protein. These results suggest that bcl-2 protein is broadly expressed in various hematopoietic neoplasms not restricted in t(14; 18) lymphomas and that germinal center cells may be involved in some arrest of bcl-2 protein expression at the posttranscriptional level.