In vitro lysis of NXS2 neuroblastoma cells by splenocytes from tumor-bearing A/J mice after treatment with fusion protein. Five days after intravenous inoculation with 5 × 10⁴ NXS2 cells, mice with established liver metastases were treated by daily (×5) intravenous injections with either 10 μg ch14.18–IL-2 fusion protein or PBS (pH 7.4), in the presence or absence of additional rmIFN-γ (300,000 IU/6 days subcutaneous via ALZET osmotic pump). Cytotoxic activity was determined 24 hours after completion of treatment in an 18-hour chromium release assay against NXS2 target cells using splenocytes at an effector-to-target-cell ratio of 100:1. Splenocytes were obtained from mice previously treated with either PBS (a), rmIFN-γ (b), fusion protein (c), fusion protein + rmIFN-γ (d), or additional anti-H2K^k antibody (25 μg/mL) in vitro (e).