Thymopentin (TP5), a 5-amino-acid polypeptide, is a safe and effective immunostimulant utilized in the treatment of a variety of immune disorders. Several residual solvents are used in the manufacture of TP5, including the "universal solvent" N,N-dimethylformamide (DMF), known for its high solubility to a variety of organic compounds. United States Pharmacopoeia has established an 880 ppm maximum concentration level for DMF to reduce potentially negative influences on the human body. This application note describes a GC/FID method for the determination of DMF utilizing a PerkinElmer Clarus® 680 GC/FID with TurboMatrix™ HS-40. The method offers a simple, sensitive and efficient means of detecting DMF with good linearity, repeatability and low reporting limits.