MoABs have demonstrated an antitumor effect and, in time, may lead to improved outcome in patients with colorectal cancer. The authors describe their experience in Sweden and summarize the results of other studies. Primarily, unconjugated mouse MoABs have been used--directed against tumor-associated antigens. More promising are hybrid antibodies composed of mouse and human elements (chimeric), or human MoABs. Such antibodies also can be used as carriers of a cytotoxic compound. The authors discuss two mechanisms by which MoABs induce their antitumor effect, and how cytokines combined with a MoAB can contribute to lysis. The potential roles of anti-idiotypic antibodies are outlined and the use of MoABs postoperatively is proposed.