Therapeutic antibodies are generally partially to fully humanized, yet they can show unwanted immunogenicity and lead to antibody response and adverse effects when administered to humans. As immunogenicity relies on a T-cell-dependent mechanism, we have evaluated in vitro the size of the preexisting CD4 T-cell repertoire specific to therapeutic antibodies in healthy donors. Specific CD4 T cells of individuals with different HLA-DR allotypes were amplified by in vitro stimulation and quantified. Well-known immunogenic proteins, KLH and a murine antibody, exhibited a strong in vitro T-cell response characterized by a mean of preexisting T cells >1 cell/10(6) cells. In contrast, the preexisting CD4 T-cell repertoires specific to 2 chimeric, 2 humanized, and 2 fully human antibodies remained generally inferior to this value, confirming the role of species-specific sequences in their shaping. Mean values ranged from 0.01 to 0.3 cell/10(6) cells and varied not necessarily in relationship with the humanization level of the therapeutic antibodies. Relationship with their known immunogenicity is discussed. These results contribute to a better understanding and prediction of immunogenicity of therapeutic antibodies in humans.