In an effort to clarify the etiology of milk allergy from the standpoint of allergen-specific immune reactions, we investigated the determinants of IgE, IgG4, and T cells specific for bovine ⍺_s1-casein from the same individual patients by using its synthetic peptides and cyanogen bromide–digested fragments. ⍺_s1-Casein is a major allergen in cow's milk, and its unique conformation enabled us to investigate the determinants of antibodies without consideration about missing the reactivities because of conformational changes. Nine patients were selected as subjects from among 129 milk-sensitive infants screened by ELISA to assess the anti-⍺_s1-casein IgE levels in their sera. By using ELISA for epitope mapping, a C-terminal region of ⍺_s1-casein was identified as a common binding site for IgE from all of these patients, whereas those for anti-⍺_s1-casein IgG4 were located in multiple regions of ⍺_s1-casein. We determined the specificities of seven ⍺_s1-casein–specific T-cell lines established from peripheral blood mononuclear cells of two of the patients. These T cells have been shown to secrete IL-4. All of the T-cell lines had different specificities to ⍺_s1-casein. However, a common amino acid residue use was found among the determinants of various T-cell lines from each patient. The results suggest that patients allergic to cow's milk have characteristic B cells recognizing a limited region of ⍺_s1-casein and secreting ⍺_s1-casein–specific IgE. These B cells may interact particularly with T cells recognizing determinants with a common structure. (J Allergy Clin Immunol 1998;101:660-71)