Intrabodies, antibodies expressed within cells, offer an interesting way to target intracellular molecules, making them potentially useful for biotechnology and medicine. However, it remains controversial whether full-size IgG intrabodies expressed in the reducing environment of the cytosol of mammalian cells are workable and structurally sound. Herein, we settle this issue with a systematic investigation of the structure and functionality of four chimeric IgG1s with distinct variable (V) domains but identical constant (C) domains. Full-size IgGs expressed in the cytosol of HEK293 cells were either assembly-competent or -incompetent, depending on the intrinsic properties of the V regions. Structural integrity of the C region is required for H:L association and the formation of a functional antigen-binding site. Partial intrachain disulfide bond formation occurs in both H and L chains of cytosolic IgG intrabodies, whereas interchain disulfide bond formation was absent and dispensable for functional assembly. IgG1s expressed in the cytosol and via the ER were shown to assemble differently. Our findings provide insight into the features and possible utilization of full-size IgGs as cytosolic antibodies in biotechnological and medical applications.