The authors used E‐rosette formation and OKT3 reactivity to determine the percent of T‐cells in lymph nodes involved by B‐cell non‐Hodgkin's lymphomas (B‐NHL) and by Hodgkin's disease (HD). The percent of helper and suppressor/cytotoxic T‐cells was determined by reactivity with OKT4 and OKT8, respectively. T‐cells were also analyzed for two signs of activation: acquisition of Ia antigens and loss of acid a‐naphthyl acetate esterase (ANAE) activity. The results were compared with those of lymph nodes exhibiting benign lymphoid hyperplasia (BLH). The percentage of T‐cells ranged from 50% to 82%, mean 63 ± 13%, in 25 cases of BLH, and from 6% to 62%, mean 23 ± 11%, in 51 cases of B‐NHL. The OKT4/T8 ratio was 1.0 to 6.2, mean 3.4 ± 2.2, in the cases of BLH, and 0.5 to 5.1, mean 2.4 ± 1.3, in the cases of B‐NHL. There was no obvious or significant correlation between the percent of T‐cells or the OKT4/T8 ratio and the surface immunoglobulin isotype expressed by the neoplastic B‐cells, the morphologic category of B‐NHL, or the clinical stage of disease. Activated T‐cells were ≦3% in the cases of BLH and B‐NHL. Fifteen lymph nodes involved by HD contained 44% to 96%, mean 74%, E+ (T) cells. Five of these 15 cases contained a significant number of E−OKT3+ cells suggesting that E‐rosette formation is not always a reliable T‐cell marker in HD. Three other cases contained a large number of E+OKT3− cells. The OKT4/T8 ratio ranged from 0.4 to 21.7, mean 6.7 ± 5.3, in these cases, representing the most significant T‐cell subset imbalances in this series. Large numbers of Ia+E+ and/or E+ ANAE− cells, presumably activated T‐cells, were present in 7 of these 15 cases of HD. These studies demonstrate the wide variation in the percent of T‐cells and in the T‐cell subset distribution in lymph nodes exhibiting benign lymphoid hyperplasia and in lymph nodes involved by B‐cell‐derived non‐Hodgkin's lymphomas and Hodgkin's disease.