In this study, a clinicopathologically and immunophenotypically diverse group of T-cell neoplasms were evaluated by one- and two-color flow cytometry and/or immunohistochemistry for the presence of eight antigens (T10, T9, IL2-R, EMA, HLA-DR, LeuM1, Ki-1, and LeuM5) which are expressed in a hierarchical manner by phytohemagglutinin (PHA)-activated benign T cells. We found that 70 of the 72 T-cell neoplasms (97%) expressed at least one of these eight T-cell activation-associated antigens (T-AAgs) and that the number and type of T-AAgs expressed by the neoplastic T cells varied according to the clinicopathologic category of T-cell neoplasia. All 5 T-cell lymphoblastic malignancies expressed T10 and T9; 2 also expressed LeuM1. Twelve of 14 (86%) T cell chronic lymphocytic leukemias (T-CLL) expressed two to four T-AAgs, most frequently T10 (86%) and HLA-DR (79%). The 26 cutaneous T-cell lymphomas (CTCL) expressed between 2 and 5 T-AAgs, most commonly T9 (92%) and HLA-DR (92%), and least often T10 (12%) and EMA (15%). Twenty-six of 27 (96%) peripheral T-cell lymphomas (PTCL) expressed more than 4 T-AAgs. Each of the T-AAgs were expressed by between 22% (LeuM5) and 85% (T9) of the PTCLs. Some T-AAgs were preferentially expressed by the PTCLs in association with other T-AAgs, such as EMA in association with IL2-R and Ki-1. In addition, LeuM5 was preferentially expressed by CD4- CD8+ T-cell neoplasms. However, only 19 of the 72 (26%) T-cell neoplasms (3/5 lymphoblastic malignancies, 3/14 CLLs, 0/26 CTCLs, 13/27 PTCLs) expressed T-AAg immunophenotypic profiles paralleling those expressed by normal peripheral blood T cells activated in vitro with PHA. These results suggest that T-AAg expression by neoplastic T cells does not often mirror the hierarchical order of expression by activated benign T cells, implying that neoplastic T cells do not usually represent the precise malignant counterpart of activated benign, normal T cells.