Activated NK cells, but not freshly isolated NK cells, lyse CRL-2598 cells. NK cells isolated from healthy blood donors were either untreated (fresh) or treated with 100 U/mL IL-2 for 3 days and then tested for their ability to lyse ⁵¹Cr-labeled CRL-2598 cells or K562 (NK-sensitive target) tumor cells. (A) Fresh NK cells were used in direct cytotoxicity assays against K562 and CRL-2598 at the E/T ratios indicated. Results from 1 representative experiment are shown; results from 2 different control subjects yielded similar results (not shown). (B) Percentage specific lysis of CRL-2598 cells was compared between IL-2–treated and fresh NK cells using an E/T ratio of 50:1. Data include results from 3 healthy control subjects. The mean and SEM of triplicate wells are shown. (C) Percentage specific lysis of CRL-2598 cells by the activated NK-cell lines, NK92 or NKL, at E:T ratios of 20:1, 10:1, 5:1, and 2.5:1. The mean and SEM of triplicate wells are shown.