OBJECTIVES: I. Determine the toxicity (detection of replication competent retrovirus) associated with CD34+ cells transduced with a retroviral vector expressing human O6-methylguanine DNA methyltransferase in adult and pediatric patients with poor prognosis CNS tumors. II. Determine the safety of genetic modification of cells carried out in the presence (ex vivo) of recombinant fibronectin (FN) fragment utilized to assist in retroviral entry into mammalian cells. III. Determine any evidence of engraftment of cells exposed to FN during retroviral transduction. IV. Determine any evidence of antibodies to FN following infusion of cells exposed to FN during ex vivo retroviral transduction.