We have previously shown that resistance to the beta-lactam mecillinam in Escherichia coli can be brought about by a high ppGpp pool, as observed under conditions of partial amino acid starvation and RelA-dependent induction of the stringent response. We show here that our E. coli wild-type strain, which is sensitive to mecillinam on minimal glucose plates, becomes resistant in the presence of L-leucine or L-serine (or cysteine, which inactivates the antibiotic). The resistance, which is not a transient effect and does not depend on the physiological state of the cells when plated, is specific for mecillinam and is reversed by the presence of isoleucine and valine in the medium. At least in the case of serine, the resistance is RelA-dependent. We conclude that the presence of leucine and serine in the growth medium cause partial starvation for isoleucine/valine, leading to induction of the stringent response and concomitant resistance to mecillinam.