This is a phase II clinical and pharmacokinetic study of suppression of human antimouse (HAMA) and antitoxin antibodies (HATA) to immunotoxin LMB-1 by Rituximab (anti-CD20). The primary objective of this study is to determine the effect of Rituximab on HAMA and HATA response to LMB-1 administered to patients with advanced carcinoma that express the B3 antigen. Other objectives include evaluation of the pharmacokinetics and anti-tumor effects.