Abstract To overcome unresponsiveness to the self-high molecular weight melanoma-associated antigen (HMW-MAA) in hosts with constitutive HMW-MAA expression, we have used as immunogen the anti-idiotypic monoclonal antibody (mAb) MK2-23, which mimics the antigenic determinant recognized by the anti-HMW-MAA mAb 763.74. In a phase I/II clinical trial, anti-idiotypic mAb MK2-23, conjugated to keyhole limpet hemocyanin (KLH) as a carrier and given with Bacillus Calmette-Guerin (BCG) as an adjuvant, elicited HMW-MAA-specific antibodies in about 60% of the immunized melanoma patients. The immune response was associated with survival prolongation. However, safety and standardization issues associated with the use of KLH and BCG in the clinical setting have prompted us to develop alternative immunization strategies. Conjugation of human interleukin 2 (IL-2) to mAb MK2-23 variable regions covalently linked to human immunoglobulin constant regions enhanced mAb MK2-23 immunogenicity in BALB/c mice to an extent similar to that induced by mAb MK2-23 conjugated to KLH and given with Freund's adjuvant. As determined by the level of serum antibodies and delayed-type hypersensitivity responses to HMW-MAA-bearing melanoma cells, immunization of mice with the MK2-23-IL-2 fusion protein elicited more robust humoral and cellular responses, respectively, than immunization with KLH-conjugated mAb MK2-23 and separate administration of IL-2. The immunogenicity of the fusion protein is dependent on IL-2 conjugation, because immunization of mice with either mAb MK2-23 or chimeric mAb MK2-23, in combination with IL-2, was not as effective in eliciting HMW-MAA-specific immune responses. These results suggest that the MK2-23-IL-2 fusion protein represents a useful immunogen to implement active specific immunotherapy in patients with melanoma, because it bypasses the requirement for KLH conjugation and adjuvant administration.