The SCID mouse model for rheumatoid arthritis (RA) is an established and reliable approach to examining the distinct mechanisms operative in RA synovium, and evaluating novel gene therapy strategies. However, serum concentrations of circulating gene therapy products following gene transfer are frequently too low to allow detection. This problem stimulated us to develop a novel implantation technique to improve the yield of these soluble gene products. Synovial fibroblasts from patients with RA were cultured, passaged, and transduced with Ad5 sTNFRp55:Ig. sTNFRp55:Ig production was confirmed by ELISA, and then cells were implanted into SCID mice using a novel implantation strategy in which pieces of human cartilage were engrafted into a fibroblast-saturated inert sponge. Thereafter, the sponges were implanted under the skin of the mice instead of under the kidney capsule, as in the original approach, allowing co-implantation of larger pieces of cartilage together with higher numbers of adenovirus-transduced RA synovial fibroblasts. The improved implantation technique not only resulted in a reduction in the number of mice needed in each experiment by approximately 60%, and a reduction of the time taken for surgery by about 50%, but also considerably enhanced the serum concentrations of the gene product sTNFRp55-Ig, allowing detection of the soluble TNF receptor p55 by standard ELISA. In summary, the improved implantation technique for the SCID mouse model for RA results in more economic animal treatment, and facilitates the detection and quantification of circulating gene products following adenovirus-based gene transfer into synovial fibroblasts. © The Japan Rheumatism Association.