An electron microscopy study of human myeloma IgG3 Kuc has shown that the hinge region in an intact molecule is in a compact state. The subunits are not fixed rigidly and are very mobile. These data are supported by results of ultracentrifugation and microcalorimetry. Non-extremal denaturating effects (pH 4.0, 20 degrees C or pH 7.8, 65 degrees C) lead to 'unfolding' of the hinge region which has a rod-like shape in electron micrographs.