Anti-idiotypic monoclonal antibodies (MAb) can mimic both protein and non-protein antigenic epitopes. In animal models, and now in patients, it is possible to induce immune responses against tumor antigens using anti-idiotypic MAb vaccines. While it is not clear how the efficacy of anti-idiotypic MAb vaccines compares with the efficacy of vaccines constructed from antigen, there are two situations where anti-idiotypic vaccines have potential advantages: (1) when the antigen is not readily available in sufficient quantities or purity, and (2) when the antigen is a non-protein. Clinical trials are underway using anti-idiotypic MAb vaccines in both of these situations.