Penicillin esters are prepd. by reaction of acid halides of penicillin (I) with an alkali in an inert org. solvent. PCl3 mixed with CHCl3 is added to I suspended in CHCl3, and pyridine is added at -2.degree.. After addn. of Et2O and recrystn. from CHCl3, the HCl of the diethylaminoethyl ester of I, m. 146-8.degree., is obtained. The HI of the diethylaminoethyl ester of I, m. 168-71.degree., the Me ester of I, and the cyclohexyl ester of I can be prepd. in a similar manner.