Peptidoglycan (PG). the major part of the cell wall (CW) of Micromonospora echinospora var 49-925, could bind about 90% of the total gentamicin (GM) absorbed on the CW and when the concentrations of PG and GM were 6 mg/ml and 500 U/ml, respectively, the amount of bound GM was approximately 88 000 U/g of PG (dry weight) and no free GM was detected in the solution. Metal ions were used to release GM from PG. Ma(2+) and Na+ restrained intensively the binding of GM at GM concentration less than 1000 U/ml. Amine could compete with GM for the binding sites as well. About 50% increase of GM titre was achieved when Mg2+ or Na+ was supplemented to the fermentation broth. (C) 2003 Elsevier Ltd. All rights reserved.