As a new type of mecillinam prodrug, mecillinam (5-substituted 2-oxo-1, 3-dioxol-4-yl) methyl esters were prepared. These esters were found to produce at least 4-fold higher mecillinam levels in blood than mecillinam itself after oral administration to mice. Mecillinam (5-methyl-2-oxo-1, 3-dioxol-4-yl) methyl ester hydrochloride was hygroscopic, but its L-tartrate was not.