Tumor-infiltrating T lymphocytes (TIL-T) are always present in B-cell-derived non-Hodgkin's lymphoma (NHL). In this investigation, we explored the possibility that collaboration might exist between these cells. TIL-T were isolated from 39 lymph nodes of patients with NHL. In most of the cases, few of them (less than 10%) possessed surface activation receptors CD25 or OKT9. In 80% of the cases, they proliferated in response to recombinant interleukin-2 (rIL-2), but the degree of proliferation was often low as compared with control populations. The influence of irradiated autologous malignant cells on the TIL-T proliferation in response to rIL-2 (40 U/mL) was also investigated: in 38% of the cases, this proliferation was not modified (group O), and in 41% it was higher (group +) and in 21% it was lower (group -). The mechanism of this immune response (specific or not) is not elucidated at present. The definition of these groups was statistically correlated with different parameters of the disease: (1) percentage of TIL-T was higher in group + (44% +/- 17%) than in group O (31% +/- 18%) and group - (24% +/- 15%); (2) B-cell proliferation in centrofollicular lymphomas was more frequently nodular or nodular and diffuse in group + (83%) and O (55%) than in group - (0%); (3) low-grade malignancies in the Working Formulation were more frequent in group + (75%) than in group O (60%) or group - (12%); (4) favorable prognosis evaluated with the Grenoble cytologic classification was more frequent in group + and O (87%) than in group - (12%); (5) actuarial survival curves showed a significantly better prognosis for patients in group +.