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From 1991 to 1995, we initiated cultures of 94 fresh tumor samples of various histologies in an effort to grow tumor-infiltrating lymphocytes (TIL) using flasks and subsequent expansion in semipermeable bags. The five most prevalent tumor types from which TIL were successfully initiated were melanoma (25 successful initiates in 34 tumor samples, 74% success rate), colorectal cancer (12 of 18, 67%), renal cell carcinoma (9 of 12, 75%), breast (4 of 5, 80%), and sarcoma (5 of 7, 71%). The overall success rate for all tumors was 67 of 94 (71%). There were no instances of contamination from the time of culture initiation through harvesting of the final cell product for clinical use. The mean number of days to reach successful initiation (> 5 X 108 cells) was 35 ± 24 days (mean ± SD). TIL were then expanded from these successful initiates for either a repeated low-dose therapy (TIL reinfusion numbers of 5 X 108–5 X 109) or for a repeated high-dose therapy (> 5 X 109–5 X 1010). The mean number of days to expand a TIL culture from the time of initiation to treatment for a first low-dose TIL was 59 days (range, 27–94 days) compared with 80 days (range, 33–209 days) for high-dose TIL. For patients who received a second or third high-dose TIL treatment, the average number of days needed to expand TIL was 39 days (n = 10) if there was no intervening cryopreservation of TIL, compared with 49 days (n = 10) if the culture had to be reestablished from cryopreserved TIL. For patients who received a second or third low-dose TIL, the mean number of days needed to expand TIL was 23 days (n = 3) if there was no intervening cryopreservation compared with 42 days (n = 17) if cultures had to be reestablished after cryopreservation of TIL. Low-dose TIL displayed predominantly CD4+ phenotype in 76% of 42 cultures, whereas high-dose TIL displayed predominantly CD8+ phenotype in 84% of 44 cultures. Cells bearing the natural killer (NK) phenotype (CD3-, CD56+) and the lymphokine activated killer (LAK) phenotype (CD3+, CD56-) were present in both low- and high-dose TIL cultures, but these phenotypes were never predominant. Cytotoxicity testing consistently demonstrated the persistence of NK and LAK activity in addition to the killing of allogeneic and autologous melanoma tumor targets. This work confirms that TIL cultures from most tumor types can be successfully established and expanded for therapeutic use, and repeated expansion from continuous TIL culture or cryopreserved TIL for repeated treatments is feasible. Such cultures are predominantly T lymphocytes that are phenotypically heterogeneous, and these phenotypes do not remain constant during prolonged time in culture.