Four children with persistent neuroblastoma after marrow ablative chemoradiotherapy and autologous bone marrow transplantation received continuous infusion of recombinant interleukin 2, 75 to 120 days after the graft. Recombinant interleukin 2 therapy did not induce any major or nonreversible toxicity, hematological toxicity in particular. One patient entered complete remission for 9 months and a second patient had a long-lasting normalization of urinary catecholamine metabolites with more than 50% regression of bone marrow metastases (8 months). In three children, recombinant interleukin 2 and a second patient entered complete remission for 9 months therapy was followed by major increase and activation of circulating natural killer cells which amounted to 80% of the circulating mononuclear cells.