I. Determine the maximum tolerated dose (MTD) of yttrium Y 90 ibritumomab tiuxetan (IDEC-Y2B8) when preceded by rituximab in children with recurrent or refractory CD20-positive lymphoma for which no autologous peripheral blood stem cell transplantation (AuPBSCT) is planned. (Group A) If the dose-limiting toxicity (DLT) in group A is purely hematological, determine the MTD of IDEC-Y2B8 when combined with rituximab, AuPBSCT, and filgrastim (G-CSF) in a second group of children with recurrent or refractory CD20-positive lymphoma. (Group B) II. Determine the DLT of rituximab and IDEC-Y2B8 in these patients. III. Determine the dosimetry of indium In 111 ibritumomab tiuxetan preceded by rituximab in these patients.