Expression of recombinant proteins in E. coli often results in formation of inclusion bodies and need to undergo renaturation. The effect of parallel feeding of oxidizing agent and denatured protein was investigated to control side oxidation and refolded yield of interferon beta-1b. Different molar ratios of iodosobenzoic acid (IBA) and interferon beta-1b (IFN beta-1b) were applied using a fed-batch process. The Reverse-Phase HPLC method was utilized to differentiate the native IFN beta-1b from the non-native oxidized and related proteins formed during the refolding process. The concentrations of IBA influenced native to non-native protein ratio hence affecting the protein quality and yield. The oxidized and related proteins were affected reversely. High concentrations of IBA increased the formation of oxidized protein while at low concentrations there was a slight increase in related proteins. The amount of 40 mu g ml(-1) of IFN beta-1b and 8 mu M of oxidizing agent was found to favor the refolding process. (C) 2010 Elsevier Ltd. All rights reserved.