In the present study, it was our intention to further the characterization of the neoplastic cells at the early stage of the T lineage, which were defined as those which bore pan-T marker(s) (CD2, CD5 and CD7) but not CD3 antigen on the surface. We studied six such cases of leukemia and two such cases of lymphoma for their phenotypes including cytoplasmic CD3 detected with flow cytometry and for the rearrangement of T cell receptor and immunoglobulin genes. The cytoplasmic expression of CD3 antigen in adult thymic cells was also studied. CD7 was expressed in seven cases, the exception being one presumably of B lineage, and rearrangements of T cell receptor gene were detected in five cases. Four cases out of these five genotypic T neoplasms had surface phenotypes compatible with the stage of thymic cells and, interestingly, they all displayed CD3 in the cytoplasm. With regard to normal cells, cytoplasmic CD3 was shown to be present only in a small population of surface CD3 negative thymic cells. These malignant cells, therefore, may have originated from such cells. The exact origins of the two cases bearing pan-T marker(s) with no rearrangement of the T cell receptor gene has not yet been determined.