A process of preparing a peptide comprising a GCC agonist sequence selected from the group consisting of SEQ ID NOs: 1-44 and 99-107, wherein the GCC agonist sequence is n amino acid units in length, with the N-terminal unit at position 1 and the C-terminal unit at position n, the process comprising: providing a first fragment having a first sequence of amino acid units from position j through position k of the GCC agonist sequence, wherein j is an integer between 1 and n -1, k is an integer between 2 and n and is greater than j, and the first fragment is protected except for an amino group of the amino acid unit at position j, or alternatively, a carboxyl group of the amino acid unit at position k, providing a second fragment having a second sequence of amino acid units from position h through position j -1 of the GCC agonist sequence or having a third sequence of amino acid units from position k +1 through position m of the GCC agonist sequence, wherein h is an integer between 1 and n -2 and is smaller than j, m is an integer between k +2 and n, and the second fragment is protected except for a carboxyl group of the amino acid unit at position j -1 or an amino group of the amino acid unit at position k +1, wherein at least one of the first and second fragments is provided via a solid-phase peptide synthesis; and coupling the first and the second fragments via a solution-phase synthesis to yield a protected peptide having a sequence of amino acid units from position h through position k of the GCC agonist sequence or a protected peptide having a sequence of amino acid units from position j through position m of the GCC agonist sequence, and wherein the peptide is purified by a process comprising adsorbing the peptide onto a polymeric adsorbent column, optionally rinsing the peptide with deionized water, eluting the optionally rinsed peptide off the polymeric adsorbent column with an isopropanol aqueous solution to form a peptide solution, removing water and the isopropanol from the peptide solution to precipitate the peptide, and optionally adding an ether to the dewatered peptide to facilitate precipitation of the peptide. providing a first fragment having a first sequence of amino acid units from position j through position k of the GCC agonist sequence, wherein j is an integer between 1 and n -1, k is an integer between 2 and n and is greater than j, and the first fragment is protected except for an amino group of the amino acid unit at position j, or alternatively, a carboxyl group of the amino acid unit at position k, providing a second fragment having a second sequence of amino acid units from position h through position j -1 of the GCC agonist sequence or having a third sequence of amino acid units from position k +1 through position m of the GCC agonist sequence, wherein h is an integer between 1 and n -2 and is smaller than j, m is an integer between k +2 and n, and the second fragment is protected except for a carboxyl group of the amino acid unit at position j -1 or an amino group of the amino acid unit at position k +1, wherein at least one of the first and second fragments is provided via a solid-phase peptide synthesis; and coupling the first and the second fragments via a solution-phase synthesis to yield a protected peptide having a sequence of amino acid units from position h through position k of the GCC agonist sequence or a protected peptide having a sequence of amino acid units from position j through position m of the GCC agonist sequence, and wherein the peptide is purified by a process comprising adsorbing the peptide onto a polymeric adsorbent column, optionally rinsing the peptide with deionized water, eluting the optionally rinsed peptide off the polymeric adsorbent column with an isopropanol aqueous solution to form a peptide solution, removing water and the isopropanol from the peptide solution to precipitate the peptide, and optionally adding an ether to the dewatered peptide to facilitate precipitation of the peptide.