The mouse anti-id MAb MK2-23 bears the internal image of the antigenic determinant defined by anti-HMW-MAA MAb 763.74. Active specific immunotherapy with anti-id MAb MK2-23 is associated with a statistically significant prolongation of survival in patients with malignant melanoma. Characterization of the immune response elicited by anti-id MAb MK2-23 may contribute to our understanding of the mechanisms underlying the apparent beneficial clinical effects of active specific immunotherapy with anti-id MAb MK2-23. Therefore, 8 HMW-MAA binding anti-anti-id MAbs elicited with MAb MK2-23 were characterized in their reactivity with a large panel of surgically removed benign and malignant tumors and of normal tissues. The 8 anti-anti-id MAbs displayed subtle differences in their immunoperoxidase staining of both benign and malignant lesions and of normal tissues. The diversity in the fine specificity of the 8 anti-anti-id MAbs is likely to reflect the few somatic mutations which occur in the amino-acid sequence of the variable regions of their heavy and light chains in the course of the immune response to MAb MK2-23. The reactivity patterns of the 8 anti-anti-id MAbs with the tissue substrates are similar, although not superimposable upon that of anti-HMW-MAA MAb 763.74 elicited with melanoma cells. This difference may reflect the imperfect mimicry by anti-id MAb MK2-23 of the antigenic determinant defined by anti-HMW-MAA MAb 763.74. © 1995 Wiley-Liss, Inc.