ABSTRACT Possible interference of vamin nutritional solution with the activity of several B-lactam antibiotics against E.coli was evaluated in vitro.In Minimal basal salts-glucose medium rapid growth inhibition of sensitive E. coli was induced by 4 µg/ml of ampicillin / cloxaillin, 8 µg/ml of ampicillin, 6 µg/ml of carbencillin, hostacillin, and cephalotin, and by 32 µg/ml of penicillin G and cloxacillin. Significant inactivation of up to 32 µg/ml of carbencillin, cephalotin, penicillin G, and hostacillin was induced by addition of 1:20 v/v vamin. This inactivation was due to the presence of specific amino acids in the mixture. Deletions of amino acids revealed that valine, leucine, isoleucine, tyrosine, tryptophan, phenylalanine, cysteine, methionine, asparagine, or threonine were essential for such inactivation while the other ami no acids (alanine, glycine, aspartic acid, argenine, glutamine, glutamic acid, histidine, serine, proline, and lycine) were not Furthermore, the presence of the three branched chain amino acids ( Valine, leucine, and isoleucine ) was essential to inactivate cephalotin and carbencillin. While the aromatic amino acids ( Tryptophan, phenylalanine and tyrosine) were essential to However, cephalotin. inactivate cloxacillin, and ampicillin / cloxacillin inactivated by any amino acid treatment ampicillin, were not These results suggest that certain B-lactam antibiotics undergo significant decrease in activity when sensitive E. coli is treated with vamin and this effect is due to the presence of specific amino acid in the solution