6 cases of post-thymic T cell lymphoma (PTCL) and 4 cases of T lymphoblastic lymphoma were investigated with a panel of 21 monoclonal antibodies (MoAb). The PTCL group was composed of 2 by 2 cases of lymphoepithelioid cell lymphoma, T zone lymphoma and T immunoblastic lymphoma. In lymphoepithelioid cell lymphoma, the cell proliferating activity was low, and a malignant clone was not identifiable by the methods used. In the cases of T zone lymphoma and T immunoblastic lymphoma, malignant populations were detectable on the grounds of immunologic marker profile combined with cytologic particularities. The 4 cases of T lymphoblastic lymphoma were of a common cortical thymic phenotype (T4+, T8 + , T6+) and presented with a mediastinal mass. In 3 of them, cell proliferation marker (Ki-67) was expressed by most tumor cells. Cell proliferating activity, however, did not inversely correlate with survival.