Five samples of tonsil, 10 reactive lymph nodes and 65 consecutive cases of non-Hodgkin lymphoma (NHL) were evaluated in suspension phenotyping with the monoclonal antibodies alpha Leu-I, alpha Leu-2a, alpha Leu-3a, OKT1, OKT3, OKT4, OKT6, OKT8, W6/32, 26/114, DA-2, 2DI, J5, AN51 and OKT9 together with conventional surface marking by rosetting (E, Fc gamma, Fc mu, C3b, C3d) and staining for surface and cytoplasmic immunoglobulin (SIg, CyIg) heavy and light chain classes. The results confirm the reproductability and specificity of staining with monoclonal antibodies against T cells and T cells subsets. Evidence is presented for reactivity of alpha Leu-I antibody with SIg positive and Ia positive cells in some lymphomas (centroblastic centrocytic, lymphocytic and immunoblastic), and 2 cases showed evidence of marking with OKT3 on SIg positive cells in T cell predominant immunoblastic lymphoma. Lymphoblastic lymphomas of T cell type expressed the marker OKT6. On the basis of these results criteria for the diagnosis of T cell lymphoma are suggested. The monoclonal antibody J5, reactive with C-ALL antigen, showed variable positivity, occasionally strong in B cells in cases of centroblastic and centrocytic follicular lymphoma. Proportions of cells staining with the monoclonal antibody OKT9 showed a correlation between levels of cellular expression of transferrin (trf) receptor and the histological grade of malignancy, OKT9+ cells being elevated in high grade lymphomas, and in some cases of transforming lymphoma of low grade histological class. These results are discussed and indicate the advantage of employing a wide range of defined monoclonal reagents in the phenotypic evaluation of NHL.