CD98 is known as a cell surface antigen expressed in proliferating normal tissues and in almost all tumor cells. Although the function of CD98 is not yet fully elucidated, it is suggested that CD98 is concerned functionally in lymphocyte activation, cell proliferation, and malignant transformation. Monoclonal antibody against human CD98 heavy chain (h.c.), termed HBJ127, shows inhibition of lymphocyte activation and tumor cell growth in vitro. These observations suggest that the epitope recognized by HBJ127 may be crucial for CD98 function. In the present study, the authors investigated the epitope recognized by HBJ127 using a phage display random heptapeptide library. Approximately 2.4 × 104-fold amplification of eluted phage titer was obtained after three rounds of panning of the phage library against HBJ127. Seven different heptapeptide sequences were isolated from 30 randomly selected clones of the post-panning phage population. A homology search using ClustalW identified the peptide sequence corresponding to 442AFS444 of human CD98 h.c. It was also found that 443F is a human-specific amino acid by comparing sequences of human, rat, and mouse origin. Reduced reactivity of HBJ127 was detected against the phenylalanine-substituted peptide but not detected against the alanine or serine-substituted one. It has been identified that HBJ127 reacts only with human species and the HBJ127 epitope position is predicted in 418–529 of human CD98 h.c. From these results and observations, it was estimated that 442AFS444 of human CD98 h.c. may be the HBJ127 epitope. Moreover, 443F may be critical for the binding of HBJ127 against human CD98 h.c. (Cancer Sci 2007; 98: 1696–1700)