INTRODUCTION Typical Laemmli gel systems, which use glycine in the running buffer, are capable of resolving proteins in the molecular mass range of ~200,000 Da down to ~3000 Da. In this protocol, the standard tricine gel system is described, in which tricine is substituted for glycine. This system permits the resolution of peptides as small as 500 Da, making it suitable for SDS-PAGE peptide mapping and for preparing samples for aminoand carboxy-terminal sequence analysis.