Fermentation broths or impure solutions containing thienamycin, a substance having antibiotic activity against gram-negative and gram-positive microorganisms, are purified using a sequence of ion exchange resins, starting with an anion exchange resin of the polystyrene-trimethylammonium type in the HCO.sub.3 -cycle, and ending with absorption on and elution from a polymeric cross-linked polystyrene-type resin adsorbent. Variations in the intermediate steps, e.g. different resins and eluates are possible and are illustrated within.