A multiple bioassay technique (FSIS‐USDA) has been evaluated with standard solutions of 38 antibiotics and sulphonamides. Activity patterns at five levels were obtained when antibiotic solutions were assayed on seven plates prepared with different media and microorganisms. A different activity pattern was obtained for the main antibiotic groups: tetracyclines,β‐lactam antibiotics, macrolides and aminoglycosides. Sulphonamides were not detected at levels under 100 μg/ml. This technique, as a post‐screening test for antibiotic residues, can be used after a screening test to identify antibiotic groups to which more specific techniques can be applied for full identification and quantification.