Methods are described for the isolation of pure crystalline 6-aminopenicillanic acid from penicillin fermentation broth. With low potency fermentation broth it was found to be convenient to prepare a concentrate of the clarified broth before proceeding with other extraction methods. Negligible amounts of 6-aminopenicillanic acid could be extracted directly into organic solvents such as butanol or butyl acetate. Partial purification could be achieved by adsorption and elution from alumina and charcoal. Adsorption and elution from a suitable ion exchange resin proved more effective. Crystalline 6-aminopenicillanic acid was then obtained by concentration of this ion exchange eluate and pH adjustment to the point of minimum solubility. Alternatively, further purification of the ion exchange eluate could be achieved by chromatography on a column of cellulose before proceeding with the crystallization step. Some properties of the isolated 6-aminopenicillanic acid are also given.