Of 12 patients with relapsed CD20(+) B-cell non-Hodgkin's lymphoma (B-NHL) enrolled in a phase I study of rituximab, 11 were eligible, and of these 2 achieved a complete response and 5 a partial response. The elimination half-life of rituximab was 445+/-361 h, and serum rituximab levels were detectable at 3 months. In a phase II study, 90 patients with relapsed indolent B-NHL or mantle cell lymphoma (MCL) were treated with infusions of rituximab 375 mg/m(2) once weekly for four doses. The overall response rate in indolent B-NHL and MCL was 61% (37/61, 95% CI 47-73%) and 46% (6/13, 95% CI 19-75%), respectively. The median progression-free survival (PFS) was shorter in MCL patients, in those with extranodal disease, and in those who had received two or more prior chemotherapy regimens ( P<0.01). Rituximab retreatment was well tolerated in 13 patients with relapsed indolent B-NHL and there were no grade 3/4 nonhematologic toxicities. Partial response was observed in five (38%, 95% CI 14-68%) patients, and the median PFS after retreatment was 5.1 months. In a single-agent phase II study of infusions of rituximab 375 mg/m(2) once weekly for eight doses against relapsed aggressive B-NHL showed, 21 (37%, 95% CI 24-51%) of the 57 eligible patients responded. In conclusion, rituximab is a highly effective agent in relapsed indolent and aggressive B-NHL and MCL with acceptable toxicities. Yttrium-90 provides advantages over iodine-131 because it delivers higher beta energy. In 2002, we initiated a feasibility study of yttrium-90-labeled ibritumomab tiuxetan for relapsed indolent B-NHL in Japan. Gemtuzumab ozogamicin (CMA-676) is a calicheamicin-conjugated humanized anti-CD33 monoclonal antibody. Of 20 patients with relapsed or refractory acute myeloid leukemia enrolled in a "bridging" phase I/II study, 7 showed an objective response. It is concluded that monoclonal antibodies will have play a significant role in the treatment of hematologic malignancies in the future.