The cross-reactive material (CRM197) of diphtheria toxin is considered to be advantageous as a carrier molecule in the formulation of a Haemophilus influenzae type b conjugate vaccine. In order to more precisely understand the function of the CRM197 in the vaccine, we have begun mapping the T-cell epitopes of the protein. A peptide which represents a segment of the primary sequence of CRM197 has been identified and found to stimulate diphtheria toxoid or CRM197-primed murine T-lymphocytes. In addition, the peptide is capable of priming T-cells in vivo for a subsequent in vitro T-cell response to itself or to the intact CRM197 molecule. The ability of the peptide to replace CRM197 as a carrier molecule was examined by immunizing mice with PRP, PRP-CRM197 conjugate, or PRP covalently coupled to the peptide. Antibodies to PRP were only detected in the PRP-CRM197 or PRP-peptide immunized groups. Both conjugates elicited primary and secondary antibody responses. Thus, a synthetic peptide representing a defined T-cell epitope of CRM197 has been functionally demonstrated based on its ability to act as a carrier molecule in a PRP conjugate vaccine.