In order to increase the solubility and antibacterial activity of ampicillin, the lysinomethylene ampicillin trihydrate was synthesized. The blood concentrations of ampicillin trihydrate and synthetic lysinomethylene ampicillin trihydrate were determined in 10 healthy male volunteers by using large plate method with Sarcina lutea ATCC 9341. The mean blood concentration of synthetic lysinomethylene ampicillin trihydrate was higher ca. 1.3 times than that of ampicillin trihydrate in 10 test subjects after oral administration.