Peptide ligands having affinity for IgG or for serum albumin are disclosed. Also disclosed are hybrid molecules comprising a peptide ligand domain and an active domain. The active domain may comprise any molecule having utility as a therapeutic or diagnostic agent. The hybrid molecules of the invention may be prepared using any of a number techniques including production in and purification from recombinant organisms transformed or transfected with an isolated nucleic acid encoding the hybrid molecule, or by chemical synthesis of the hybrid. The hybrid molecules have utility as agents to alter the elimination half-times of active domain molecules. Elimination half-time is altered by generating a hybrid molecule of the present invention wherein the peptide ligand has binding affinity for a plasma protein. In a preferred embodiment, a bioactive molecule having a short elimination half-time is incorporated as or into an active domain of the hybrid molecules of the invention, and the binding affinity of the peptide ligand domain prolongs the elimination half-time of the hybrid as compared to that of the bioactive molecule.