Interleukin-2: prospects for lymphocyte-mediated destruction of pediatric malignancies. Immunologic therapy of cancer was speculated on at the turn of the century. In animals, in vitro, and most recently in patients, irrefutable evidence has been obtained that lymphocyte responses can have a reproducible and beneficial antitumor effect. These results indicate that "biologic response modification" may truly become a fourth modality for cancer treatment, to be integrated into the standard approaches of radiation, chemotherapy, and surgery. To what extent these immune approaches may enable eradication of microscopic amounts of residual diseases in children who would otherwise have a recurrence of their malignancies remains the critical issue for testing over the next decade. Enthusiasm regarding this approach is abundant, but critical evaluation of all clinical trials is essential to best focus these mechanisms into effective therapy. 