To establish the maximum tolerated dosage (MTD), the dose-limiting toxicities (DLTs), and pharmacokinetic parameters of CI-980, a novel tubulin binder, in children with solid tumors refractory to standard therapy. Patients 21 years of age or younger with adequate nutritional, hematopoietic, renal, and hepatic function were eligible. The patient must not have been pregnant. Patients with brain tumors were not eligible for any dosage level until it was demonstrated the level did not produce DLT in patients with extracranial solid tumors. The starting dosage level was 3.5 mg/m2/day, for 3 days, administered as a continuous intravenous infusion (80% of the adult MTD). If a dosage level was associated with dose-limiting myelotoxicity, growth factors were to be added. Thirty-three patients received CI-980. Twenty-four had solid tumor; 9 had brain tumor. The MTD achieved without granulocyte colony stimulating factor (G-CSF) was 3.5 mg/m2/day (DLT: neutropenia) and with G-CSF, it was as follows: patients with brain tumor, 4.2 mg/m2/day (DLT: myelosupression); and patients with solid tumor, 5 mg/m2/day (DLT: cortical toxicity). Several responses were seen, most notably prolonged stable disease in two of five patients with medulloblastoma. Pharmacokinetic data showed a mean steady state level of 1.74 ng/mL for two patients treated with the 5 mg/m2/day regimen, with rapid decay after the termination of the infusion. CI-980 showed preliminary evidence of activity in recurrent pediatric malignancies, with tolerable, reversible toxicities.