Stability of D-I preparation on standing at low temperature was examined. Standing in saline solution at 4°, the hypotensive activity was stable for the first 45 days, and lost completely within a following week. Standing as CHCl₃-MeOH (2 : 1, v/v) solution at 4°, the decomposition began at 75 days later, and it reached a plateau maintaining about 20% of the initial activity. At-20°as a syrup form, the activity could be stored unaltered for 120 days. In the partition experiments, n-hexane and ethylether could extract little active component at all pH values tested (pH 4, 7 and 10). While n-butanol or Folch's solvent could extract unilaterally without any pH effects. The nuclear magnetic resonance spectrum of D-I strongly resembles that of lysophosphatidyl choline (LPCh). Specific optical rotation was also similar to that of LPCh. In the assessment of molecular weight on Sephadex LH-20 column eluting with 80% EtOH, the active substance was eluted in the neighbourhood of L-dipalmitoylphosphatidyl choline or L-1-palmitoyl-LPCh and its molecular weight was estimated to be about 500 to 800. In the analysis, the ratio of phosphorus, glycerol, choline and fatty acid was approximately 1 : 1 : 1 : 1, and oleic acid shared about half of the fatty acid composition. D-I might be a LPCh or closely related compound to it.