PATIENT CHARACTERISTICS: Age: 2 to 21 Performance status: Karnofsky 50-100% (over 10 years of age) Lansky 50-100% (under 10 years of age) Life expectancy: At least 8 weeks Hematopoietic: Absolute neutrophil count greater than 1,000/mm3 Platelet count greater than 50,000/mm3 Hemoglobin greater than 7.0 g/dL Transfusion support allowed after bone marrow transplantation or extensive radiation Hepatic: Bilirubin less than 1.5 mg/dL SGPT less than 2 times normal Renal: Creatinine no greater than 1.5 mg/dL OR Creatinine clearance greater than 60 mL/min Other: Not pregnant or nursing Fertile patients must use effective contraception during and for 6 months after study No significant systemic illness including infections No amino acidurias or organic acidemias