Ffteen cases of peripheral T-cell lymphoma were studied to evaluate the respective properties of various histologic types using enzyme histochemical and ultrastructural examinations in addition to immunological methods. Eleven cases in an ATLA negative group manifested various histologic patterns such as IBL-like, pleomorphic and Lennert's lymphomas in comparison with the relatively monomorphic proliferation of neoplastic lymphoid cells in the 4 ATLA-positive cases. The presence of neoplastic clear cells is characteristic of peripheral T-cell malignancies, and is likely to be found in CD4+ lymphomas. There is an occasional reaction of epithelioid histiocytes and plasma cells with eosinophils, the former being designated Lennert's lymphoma and the latter IBL-like Tcell lymphoma. Immunological examination revealed four immunophenotypic patterns : (1) CD2+3+4+8-, (2) CD2+ 3-4+8-, (3) CD2+3+4-8+, and (4) CD2+3+4+8+, but did not provide information concerning the intimate relationship between histologic types and immuno-phenotyes. p-Glucuronidase reactivity, however, contributed to the distinction between helper and suppressor T-cell malignancies, suggesting its usefulness for distinguishing these two cell types and their malignant counterparts.