The number and distribution of reactive cells in more than 100 non-Hodgkin lymphomas (NHL) were evaluated in situ in cryostat sections. The results were compared with histological and clinical findings. 40% of the T cell content of normal lymphatic tissues was found in tissues of B-cell NHL. This corresponds to a mean of 2 X 10(4) reactive T-cells/microliter tumour tissue. The numerical density of natural killer (NK) cells within tumours was similar to that of normal lymphatic tissues (0.5 X 10(4) cells/microliter). The distribution of the reactive cells within the tumours was diffuse except in the case of centroblastic/centrocytic lymphomas. On evaluation of the different histological entities a significant correlation was obtained between number of helper/inducer (TH) cells, TH:T-suppressor (Ts) ratio and prognostically favourable histological subgroups. Furthermore, independent of histological criteria, a close correlation was found between a high number of TH-cells, a high TH:Ts ratio and a favourable clinical course (p less than 0.04). NK-cell infiltration was present to a markedly higher extent in tissues of patients with generalized disease as compared with localized disease (p less than 0.03). Low-grade malignant NHL contained significantly more NK cells than high-grade malignant NHL, as was the case also in treated, as opposed to untreated patients. These findings together suggest that reactive cells influence tumour growth via local interactions. However, tissue distribution and infiltration density of T-cell subpopulations and NK cells were clearly different. Thus, in tumour tissues different and independent immunoregulatory mechanisms seem to be associated with these two lymphocyte subsets.