The total T cell population and T cell subsets in ten lymph nodes with reactive lymphoid hyperplasia (RLH) and 23 specimens (21 lymph nodes, one stomach, and one small bowel) involved by histologically and immunohistologically diagnosed B cell non-Hodgkin's lymphomas (NHLs) were determined by reactivity with monoclonal antibodies Leu 4-CD3, Leu 3-CD4, and Leu 2-CD8 in cytospin preparations from cell suspensions. T cell populations were also investigated for the coexpression of dipeptidylaminopeptidase IV (DAP IV) activity, which was visualized simultaneously with cell surface immunostaining by a combined cytochemical and immunocytochemical method. The mean absolute percentage of Leu 4-CD3+ (total T) and Leu 3-CD4+ cell populations was significantly lower in B cell NHL cases than in RLH cases (35% v 54%, P less than .001; 29.5% v 44.4%, P less than .01). No difference in the mean absolute percentage of the Leu 2-CD8+ T cell subset was found between the RLH cases and the B cell NHL cases classified as other than category A as described by the Working Formulation (WF) of NHLs. The relative percentage of Leu 4-CD3+ and Leu 2-CD8+ cells coexpressing DAP IV reactivity was lower in B cell NHL cases than in RLH cases (27.3% v 39.5%, P less than .05; 13.5% v 24.4%, P less than .10). There was no difference in the proportion of Leu 3-CD4+ cells expressing DAP IV reactivity between the NHL and RLH groups (34.5% v 36.1%). Since the mean relative percentage of Leu 2-CD8+ cells expressing DAP IV reactivity in the B cell NHL group in the other than category A according to the WF was lower than that of the RLH group (12.5% v 24.4%), and whereas the mean absolute percentage of total Leu 2-CD8+ cells was similar in the two groups (16.6% and 16.6%), a possible defective role of this Leu 2-CD8+ DAP IV+ subset, at least in B cell NHLs in the other than category A according to the WF, may be hypothesized.