Two major contaminating proteins of the cytoplasmic inclusion bodies in recombinant Escherichia coli overexpressing a LacZ-papain fusion protein were studied. The proteins were isolated and their amino acid composition and N-terminal sequence analyses were performed. The N-terminal sequence of these proteins were then compared with the sequence of E. coli proteins available in the data bank. The sequences of the contaminating proteins matched with the outer membrane protein II and outer membrane protein F of E. coli.