VP-16-213, a semisynthetic podophyliotoxin, was tested for antitumor and clinical toxicity in 126 children. The drug was administered iv daily x 5 days every 2 weeks at a starting dose of 75 mg/m2/day. The dose was increased by 25 mg/m2/day/course until clinical response or significant toxicity occurred. The only major toxicity was hematologic, with neutropenia as the most predominant feature. There was one local allergic reaction at the site of injection. No systemic allergic responses were reported. The drug demonstrated significant activity in acute myelomonocytic leukemia with four responses among 19 patients, less activity in acute myelocytic leukemia with two responses among 44 patients, and little activity in acute lymphocytic leukemia with only one partial response among 12 patients. Objective partial responses occurred in ten of 48 patients with solid tumors: two each with Wilms' tumor, lymphoma, and histiocytosis X, and one each with rhabdomyosarcoma, neuroblastoma, Ewing's sarcoma, and undifferentiated carcinoma. The inclusion of VP-16-213 in combination chemotherapy for childhood acute myelomonocytic leukemia and acute myelocytic leukemia appears indicated in patients relapsing after initial therapy. For solid tumors this is an interim report, with further patient accrual required before specific comments can be made.