Abstract Three patients with triose phosphate isomerase deficiency have been studied. 1. (a) A profound deficiency was found in red blood cells, white blood cells, and fibroblasts of the patients. 2. (b) Extracts from leukocytes and fibroblasts showed a marked decrease in thermal stability. 3. (c) The electrophoretic mobility of the major band was normal in the three cases. However, the fast electrophoretic band, found in extracts of normal fibroblasts, was absent in the patients. 4. (d) An immunological study was performed using Mancini's immunodiffusion technique. No cross-reacting material was found. The TPI deficiency in all three cases seemed to result, not from a reduced specific activity of the enzyme molecule, but from a marked instability of the enzyme.