C18 has been the traditional choice for purification of peptides, but recent work has trended toward the use of C4. The processes of peptide synthesis involve small molecules and produces by products that are also small molecules. These compounds partition rather than adsorb to the surface of C18. Partitioning moecules will elute much later on C18 than on C4. Larger molecules, such as peptides (>5mer) and proteins, adsorb rather than partition. These by-products with may coelute with peptides on C18 will be widely separated on C4. Peptide elution changes very little. PRABHUSANKAR, RESEARCH OFFICER SHASUN DRUGS AND CHEMICALS,CHENNAI