Tumor-infiltrating T lymphocytes (TIL-T) were quantitated by three color flow cytometry in cell suspensions from excisional biopsy specimens of 43 B cell non-Hodgkin's lymphomas (NHL) and 8 benign lymphoid hyperplasias (BLH) to identify potential differences in host T cell responses. We quantitated three TIL-T subsets: CD3 + CD4 + CD8- (helper-inducer), CD3 + CD4-CD8 + (suppressor-cytotoxic) and CD3 + CD25-HLADr+ (long term activated TIL-T) and compared them in three diagnostic groups: BLH, low grade B cell NHL (LG NHL) and intermediate-high grade B cell NHL (IG-HG NHL). The following results were obtained: Mean percentage±s.e. of activated TIL-T for BLH, LG-NHL and IG-HG NHL: 10.3±1.9, 23.2±4.6 and 38.8±9.5, respectively. Mean percentage±s.e. of suppressor-cytotoxic TIL-T for same groups: 13.9±1.5, 14.9±1.9 and 34.4±4.5, respectively. Mean percentage±s.e. of helper-inducer TIL-T cells for the same groups was 38.2±12.7, 32.1±7.2 and 22.5±4.6, respectively. Helper/suppressor ratio±s.e. for same groups was 3.0±1.1, 2.4±0.6 and 1.3±0.4, respectively. Activated and suppressor-cytotoxic TIL-T percentage progressively increased from BLH toward IG-HG NHL. The percentage of these two TIL-T subsets were significantly higher in IG-HG NHL than in BLH and LG-NHL (P<0.0007, 0.0002, 0.0001 and 0.0260 for the comparisons TIL-T in BLH vs IG-HG NHL and LG-NHL vs IG-HG respectively). These results suggest the possibility that activated-cytotoxic TIL-T represent a host cellular immune response versus neoplastic NHL B cells and encourage future in vitro functional studies.