The cytotoxicity against neuroblastoma cells of IL-2-activated peripheral blood (PBL) and tumor-infiltrating lymphocytes (TIL) was evaluated in seventeen patients with neuroblastoma. Regional lymph node lymphocytes (LNL) were similarly studied in some patients. Three allogeneic neuroblastoma cell lines, LA2D2, LA2B4 and SIFA, established from the different metastases of the same patient were used as targets. Of the three neuroblastoma lines, LA2D2, with low CD56 expression, was the most susceptible to IL-2-activated lymphocytes, while SIFA, with high CD56 expression, was resistant in the greatest degree. LA2B4 showed moderate susceptibility. Although TIL (73.9 +/- 2.1%), LNL (81.0%) and PBL (76.2 +/- 3.1%) revealed similar cytotoxic activity to K562, they demonstrated distinct cytotoxic activities to each neuroblastoma cell line, as follows: against LA2D2: TIL 56.3 +/- 4.2%, LNL 52.1%, PBL 33.6 +/- 4.9% (P < 0.01); against LA2B4: TIL 47.3 +/- 3.3%, LNL 37.8%, PBL 33.7 +/- 4.8% (P < 0.05); against SIFA: TIL 27.0 +/- 6.2%, LNL 20.7%, PBL 13.9 +/- 2.4% (P = 0.056). TIL always showed higher cytotoxic activity against neuroblastoma cells than those of LNL and PBL, whereas LNL were more cytotoxic than PBL. This data showed that TIL from neuroblastoma patients preferentially killed neuroblastoma cells. It was suggested that lymphocytes in the tumor site and regional lymph node could have been sensitized with neuroblastoma-related antigens and exert preferential killing activity against neuroblastoma cells. Phenotypical analysis revealed that TIL had a larger population of CD56+ cells than PBL. Conversely, PBL had a higher population of CD16+ cells than TIL. The cytotoxic activity of TIL significantly decreased by the depletion of CD56+ cells (10.9 +/- 6.2 from 49.9 +/- 5.9% against LA2D2, P < 0.001). These results indicated that CD56+ cells were most responsible for the killing of neuroblastoma cells, and that TIL, with a high proportion of CD56+ cells with strong activity, would be the best source for the immunotherapy of neuroblastoma. Additionally, since neuroblastoma cell lines used in the present study were derived from the different metastases of the same patient, heterogeneity in the susceptibility to lymphocytes might result from the differential expression of tumor-related antigens on these cell lines.