These studies were undertaken to investigate the stability of lysinomethylene ampicillin in various solution such as pH solutions made of 0.1N NaOH∼0.1N HCl, 0.1N Na₂HPO₄∼0.1N Citric acid and aqueous solution, artificial gastric juices, intestinal juices, used of spectrophotometer at 320㎚. The results were as follows; 1. Lysinomethylene ampicillin in the various pH solution made of 0.1N NaOH 0.1N HCl (pH1∼10) was decomposed below to pH 1 and showed the best stability at pH 5. 2. In the various pH solutions with 0.1N Na₂HPO₄∼0.1N Citric acid (pH 2-9) lysinomethylene ampicillin was decomposed below to pH 4, and showed the best stability at pH 7. 3. Lysinomethylene ampicillin the various pH solution with the artificial gastric juices and intestinal juices was more stabled than that of ampicillin.