Recombinant protein production system using transgenic rice grain offers many advantages in higher accumulation, preservation, lower production cost, ease of scale up and low risk of contamination by toxic materials. We developed a transgenic rice strain whose seeds accumulate human interleukin (IL)-10, a cytokine that suppresses inflammation-related immune responses. We also developed a method of extracting and purifying IL-10 from rice seeds. A biochemical crosslinking method was used to detect the biologically active noncovalent dimer of IL-10. This method was useful for developing efficient methods of refolding and purification. The purified IL-10 comprised only noncovalent dimers and showed higher activity than the commercial IL-10. The purified IL-10 had very low endotoxin contamination and is expected to have broad clinical application. (C) 2010 Elsevier Inc. All rights reserved.