Myeloma cells secrete monoclonal immunoglobulin (Ig), called myeloma protein. The variable (V) regions of myeloma proteins are unique to each plasma cell tumor, and therefore contain highly tumor-specific antigenic determinants called idiotopes (Id). In ongoing clinical trials, myeloma patients are vaccinated against the Id of their own myeloma protein. T cells with specificity for Id are thought to be of importance in eradication of multiple myeloma. We have developed a mouse model to study the molecular and cellular mechanisms for how Id-specific T cell protect against myeloma. A T cell receptor (TCR) transgenic mouse model has been established. CD4+ T cells in these mice recognize a particular Id-peptide presented on a MHC class II molecule. The Id-peptide represents an 11-mere of the variable region L chain of a particular mouse myeloma, MOPC315. TCR-transgenic mice are protected against a challenge with MOPC315 cells. Id-specific CD4+ cells protect in the absence of anti-Id antibodies. Dendritic cells in s.c. tumors take up myeloma protein and present Id-peptide on class II molecules to CD4+ cells which become activated and then kill bystander myeloma cells by an unknown mechanism. When TCR-transgenic mice are injected with large amounts of MOPC315 myeloma cells, resistance is overcome and some 30% of mice develop tumors. In these mice, Id-specific T cells become deleted as the myeloma protein serum concentration exceeds 50 microg/ml. In conclusion, in an experimental model, Id-specific CD4+ T cells protect against myeloma. However, once a tumor is established, Id-specific T cells become incapacitated. Based on these results, it is suggested that Id-vaccination in humans should be reserved for eradication of minimal residual disease, eg after high dose chemotherapy and stem cell transplantation.