The Gd3+-induced relaxation rate enhancements in the 13C nuclear magnetic resonance (NMR) spectrum of the immunoregulatory peptide thymopentin (arginine-lysine-asparatic acid-valine-tyrosine) were examined. This peptide, which corresponds to positions 32-36 of the thymic hormone thymopoietin, retains the biological activity of the parent hormone, i.e. induction of selective differentiation of T lymphocytes. The relaxation rate enhancements experienced by 13C nuclei were analyzed to obtain estimates of average 13C-Gd3+ distances in the lanthanide ion complex of the peptide. These results, together with 1H NMR investigations of the lanthanide complexes of this peptide, suggest that both the aspartyl and tyrosyl carboxylate groups bind to the metal ion in a chelate fashion. These data provide some of the boundary conditions necessary to construct detailed molecular models of the Ln3+ (Ln lanthanide) complexes of the immunoregulatory peptide. © 1983.