Four member institutions of the Southeastern Cancer Study Group (SECSG) investigated 27 cases of malignant lymphoma proved to be of T-cell origin by a frozen section immunoperoxidase technique. The specimens were sent to one central laboratory in Michel's transport medium, where phenotyping studies were performed with a large number of monoclonal antibodies. The phenotypes encountered differed as a group from that reported for lymphoblastic lymphoma, but there was significant diversity within the peripheral T-cell lymphomas. Most tumors were of a mature helper/inducer phenotype (Leu-3+, Leu-2-), but nine of the 27 lymphomas expressed Leu-3 and Leu-2 in other combinations. Half of the lymphomas expressed abnormal T-cell phenotypes in that one or more pan-T-cell markers usually present in nonneoplastic T-cell proliferations were absent. Antibody 3A1 was the pan-T marker that was most frequently lacking in the peripheral T-cell lymphomas. The tumors were also studied for their expression of three markers associated with T-cell activation--HLA-DR, transferrin receptor, and interleukin 2 receptor. The majority of the lymphomas expressed one or more activation markers. However, these three markers appear to be expressed independently. In general, there was no simple correlation between the phenotype of the tumor and the histologic appearance, although neoplasms of morphologically higher grades were somewhat more likely to express T-cell activation markers.