The development of hybridoma technology by Köhler and Milstein revolutionized the field of antibody research, paving the way for the creation of monoclonal antibodies. Due to their ability to specifically bind target molecules via antigen–antibody reactions, antibodies have become a major focus in therapeutic development. However, antibodies obtained from immunized animals such as mice may cause immunogenicity or side effects when administered to humans. While humanized mice offer a solution, their high-cost limits widespread adoption. Advances in genetic engineering technology have made it possible to create chimeric antibodies or humanized antibodies and have greatly reduced immunogenicity. In this chapter, we explain in detail a laboratory-scale protocol to produce recombinant IgG antibodies at the milligram scale, which is essential for in vivo research, using transient expression in CHO cells.