T-cell determinants of bovine beta-lactoglobulin (beta-LG) in BALB/c(H-2d), C57BL/6(H-2b) and C3H/He(H-2k) mice were identified using a set of overlapping synthetic peptides encompassing the entire primary structure of the protein. Lymph node cells from mice immunized with beta-LG were subjected to cell proliferation assay in the presence of these peptides and uptake of 3H-labeled thymidine was measured. Determinant regions were indicated to lie in residues 42-56, 62-76 and 139-153 in BALB/c mice, residues 11-26, 72-86, 100-113 and 119-133 in C57BL/6 mice, residues 72-86, 91-104, 129-143 and 139-153 in C3H/He mice. Some of these fragments included the antigenic motifs predicted by hypotheses according to amphipathicity and sequential patterns of peptides. We reported elsewhere that residues 42-56 and 72-86 represent one of the B-cell antigenic determinants in BALB/c and C3H/He, respectively. These peptides serve as good models of colinear T- and B-cell determinants as they contain both of T- and B-cell determinants.