indicating that beta-lactam ring opening is much slower than esterhydrolysis. The O-(N-alkylamido)methylesters of penicillin G displayed similar in vitro antibacterial activity to penicillin G itself. CONCLUSIONS Compared to the penicillin G derivatives, the much higher stability of the O-(N-methylbenzamido)methyl benzoate, acetate and valproate esters (which gave rise to a Bronsted Beta 1g value of ca. -1)