Recombinant human interleukin-1 receptor antagonist (rhIL-1ra) was chemically conjugated with succinimidyl carbonate monomethoxyl polyethylene glycols of 5 kDa (SC-PEG(5k)) and 10 kDa (SC-PEG(10k)) molecular weight. A facile purification of the conjugates was achieved by one-step cationic exchange chromatography. The purity of mono-PEGylated protein was greater than 95%. The purified conjugate was characterized by multi-angle laser light scattering (MALLS) for determining the apparent gyration radius (r(g)) and hydrodynamic radius (rh). MALLS results showed that the conjugation of PEG markedly enhanced r(g) and r(h) of parent protein (r(g): from 15.7 to 48.2 nm for the PEG(5k) and 81.9 nm for the PEG(10k); r(h): from 4.2 to 5 8.4 nm for the PEG(5k) and 102.3 nm for the PEG 100. The PEGylated rhIL-1ra retained 44.6% of binding activities to the cell receptor for PEG(5k) and 40.2% for PEG(10k), compared to the original protein. (C) 2007 Elsevier Ltd. All rights reserved.