Recombinant human interleukin-1 receptor antagonist (rhIL-1RA) is an important cytokine in the treatment of inflammatory diseases. However, it is instable in aqueous solution and prone to degrade without the addition of any excipient. Following the 30- or 60-day storage in 50 mM sodium acetate (pH 5.0) at room temperature, rhIL-1RA markedly degraded into three species (denoted as P1, P2 and P3 in this study), the bioactivities of which to a different extent was lost (from 9.72 x 104 Ul/mg to 3.07 x 10(3) Ul/mg for P1, 5.49 x 10(3) Ul/mg for P2, 1.09 X 10(4) Ul/mg for P3, respectively). To solve this problem, we prepared the mono-PEGylated rhIL-IRA with propionaldehyde mPEG (ALD-PEG, M(w) 5000 Da). The conjugate showed more favorable stability than original protein, and remained homogeneous under the similar storage conditions. In addition, the activity of the conjugate was well retained (from 5.80 x 10(4) Ul/mg to 4.92 X 10(4) Ul/mg), compared to that of original protein. The results based on the combination analysis of CD, ion exchange chromatography and RP-HPLC, revealed that the stability improvement of rhIL-1RA majorly benefited from the PEG strands protection against the protein conformational changes occurred during the storage. (C) 2009 Published by Elsevier Ltd.