<jats:p> <jats:italic>Streptomyces lipmanii</jats:italic> L100 produces two β-lactam ring-containing antibiotics: penicillin N and 7-(5-amino-5-carboxy valeramido)-7-methoxy-cephalosporanic acid. These antibiotics are modified tripeptides derived from α-aminoadipic acid, cysteine, and valine. Mutants defective either in the biosynthesis or in the control of amino acids belonging to the aspartic and pyruvic acid families were isolated and characterized with regard to their effect on antibiotic synthesis. In a number of these mutants antibiotic synthesis was either completely inhibited or increased up to threefold over wild-type levels.