PENICILLIN has been obtained in the form of a highly purified barium salt by repeated fractional extraction from amyl acetate into water, chromato-graphic separation on an alumina column, treatment of the active fraction with aluminium amalgam and further repeated chromatographic separation until the alumina column appeared homogeneous. The preparation thus obtained, though not crystalline, has an activity of 450-500 Oxford penicillin units per mgm., corresponding to a complete inhibition of the growth of Staphylococcus aureus in broth in a dilution of 1:25,000,000. Penicillin must therefore be regarded as one of the most powerful antibacterial substances with predominantly bacteriostatic action known.