Twenty-three T-cell neoplasms were divided, according to their reactivity with the OKT monoclonal antibodies as follows: Fourteen neoplasms of diverse histopathology expressed the OKT3+ OKT4+ phenotype, commonly associated with the helper T-cell subset; seven histologically similar lymphoblastic neoplasms expressed diverse phenotypes consistent with various stages of intrathymic differentiation and two neoplasms expressed the uncommon OKT3+ OKT10+ phenotype. Thus, T-cell neoplasms are divisible into phenotypes that correspond to normal stages of T-cell differentiation and functionally distinct T-cell subsets. Benign and malignant lymphoid cells were investigated in cell suspension and in cryostat tissue sections for their reactivity with OKB1, OKB2, OKB4, and OKB7, monoclonal antibodies known to detect distinctive B lymphocyte antigens. Fetal liver pre-B cells, cases of pre-B acute lymphoblastic leukemia, and common-type acute lymphoblastic leukemia were OKB2+ but unreactive with the other OKB antibodies. All mature lymphoid tissue B cells and 47/47 surface immunoglobulin (SIg)-positive B-cell neoplasms were OKB2+. Interfollicular, follicular center, and many, but not all, mantle zone B cells were OKB1+ OKB7+. Follicular center B cells were OKB4+ but mantle zone and interfollicular B cells were OKB4-. 45/47 SIg+ B-cell neoplasms were OKB1+ OKB4+. 45/46 SIg+ B-cell neoplasms were OKB7+. Benign and myeloma plasma cells were OKB-. T-cell neoplasms were OKB2- OKB4- but were occassionally OKB1+ and OKB7+. The OKB antibodies appear to detect distinctive antigens that may be expressed at different stages of B-cell differentiation.