6-Aminopenicillanic acid represents the nucleus of the penicillin molecule and the antibacterial activity of a given penicillin depends on the nature of the side chain attached to this molecule. 6-Aminopenicillanic acid itself shows a relatively low order of antibacterial activity. It is a substrate for penicillinase and an inducer of penicillinase formation. 6-Aminopenicillanic acid is produced in penicillin fermentations when a precursor for the side chain is omitted and can also be obtained by enzymatic removal of the side chain of penicillin. 6-Aminopenicillanic acid provides a starting material for the preparation of new and clinically valuable penicillins.